I can confirm that all data is not retained. After the sync has completed, I tested by downloading an image from the NextCloud data directory to my PC using FTP. Also using FTP, I downloaded the original image from my phone. The original image from my phone contains the GPS data, while the copy that came from the NextCloud server had the fields still there, but they were all zeroed out.
The fact that all the information fields are still there makes me think the phone sync software is deliberately doing that, rather than simply not uploading it. A backup solution like nextcloud should never alter the original file.
You say that all your data is retained? Could you post your setup info - version you are using, and maybe, if possible, the GPS Exif data from one of your images? It is possible that the sync client is removing particular tags, and yours is different to mine.
My exif from the Phone is:
ExifToolVersion : 12.68
[System] FileName : IMG_20231113_112033-Phone.jpg
[System] Directory : .
[System] FileSize : 8.2 MB
[System] FileModifyDate : 2023:11:13 11:20:33+11:00
[System] FileAccessDate : 2023:11:14 05:01:22+11:00
[System] FileInodeChangeDate : 2023:11:14 05:01:22+11:00
[System] FilePermissions : -rw-r--r--
[File] FileType : JPEG
[File] FileTypeExtension : jpg
[File] MIMEType : image/jpeg
[File] ExifByteOrder : Big-endian (Motorola, MM)
[File] ImageWidth : 4000
[File] ImageHeight : 3000
[File] EncodingProcess : Baseline DCT, Huffman coding
[File] BitsPerSample : 8
[File] ColorComponents : 3
[File] YCbCrSubSampling : YCbCr4:2:0 (2 2)
[IFD0] Artist : Richard Pillay
[IFD0] Make : samsung
[IFD0] ImageHeight : 3000
[IFD0] Orientation : Rotate 90 CW
[IFD0] ModifyDate : 2023:11:13 11:20:33
[IFD0] YResolution : 72
[IFD0] XResolution : 72
[IFD0] ImageWidth : 4000
[IFD0] Model : SM-G998B
[IFD0] Software : G998BXXS9EWJO
[IFD0] YCbCrPositioning : Centered
[IFD0] ResolutionUnit : inches
[GPS] GPSLatitude : 33 deg 50' 29.31"
[GPS] GPSAltitude : 37.1 m
[GPS] GPSLatitudeRef : South
[GPS] GPSAltitudeRef : Above Sea Level
[GPS] GPSProcessingMethod : CELLID
[GPS] GPSLongitudeRef : East
[GPS] GPSTimeStamp : 00:20:33
[GPS] GPSLongitude : 151 deg 3' 13.96"
[GPS] GPSDateStamp : 2023:11:13
[GPS] GPSImgDirection : 120.81
[GPS] GPSImgDirectionRef : Magnetic North
[ExifIFD] ExifVersion : 0220
[ExifIFD] ApertureValue : 1.8
[ExifIFD] ExposureProgram : Program AE
[ExifIFD] UserComment : Yaw:120.81007,Pitch:-20.26522050904004,Roll:86.95352588209022
[ExifIFD] ColorSpace : sRGB
[ExifIFD] ImageUniqueID : XA8XLNF00SM XA8XLNF00SM.
[ExifIFD] ISO : 50
[ExifIFD] ComponentsConfiguration : Y, Cb, Cr, -
[ExifIFD] MaxApertureValue : 1.9
[ExifIFD] FocalLengthIn35mmFormat : 24 mm
[ExifIFD] SubSecTimeDigitized : 679
[ExifIFD] DateTimeOriginal : 2023:11:13 11:20:33
[ExifIFD] SubSecTimeOriginal : 679
[ExifIFD] WhiteBalance : Auto
[ExifIFD] CreateDate : 2023:11:13 11:20:33
[ExifIFD] MeteringMode : Center-weighted average
[ExifIFD] FocalLength : 6.7 mm
[ExifIFD] ExposureTime : 1/50
[ExifIFD] Flash : Unknown (0x2)
[ExifIFD] SceneCaptureType : Standard
[ExifIFD] SubSecTime : 679
[ExifIFD] FNumber : 1.8
[ExifIFD] LightSource : Unknown
[Composite] Aperture : 1.8
[Composite] ImageSize : 4000x3000
[Composite] Megapixels : 12.0
[Composite] ScaleFactor35efl : 3.6
[Composite] ShutterSpeed : 1/50
[Composite] SubSecCreateDate : 2023:11:13 11:20:33.679
[Composite] SubSecDateTimeOriginal : 2023:11:13 11:20:33.679
[Composite] SubSecModifyDate : 2023:11:13 11:20:33.679
[Composite] GPSAltitude : 37.1 m Above Sea Level
[Composite] GPSDateTime : 2023:11:13 00:20:33Z
[Composite] GPSLatitude : 33 deg 50' 29.31" S
[Composite] GPSLongitude : 151 deg 3' 13.96" E
[Composite] CircleOfConfusion : 0.008 mm
[Composite] FOV : 73.7 deg
[Composite] FocalLength35efl : 6.7 mm (35 mm equivalent: 24.0 mm)
[Composite] GPSPosition : 33 deg 50' 29.31" S, 151 deg 3' 13.96" E
[Composite] HyperfocalDistance : 2.97 m
[Composite] LightValue : 8.3
The same image from the NextCloud storage is:
[ExifTool] ExifToolVersion : 12.68
[System] FileName : IMG_20231113_112033-NextCloud.jpg
[System] Directory : .
[System] FileSize : 8.2 MB
[System] FileModifyDate : 2023:11:13 11:20:33+11:00
[System] FileAccessDate : 2023:11:14 05:03:19+11:00
[System] FileInodeChangeDate : 2023:11:14 05:03:13+11:00
[System] FilePermissions : -rw-r--r--
[File] FileType : JPEG
[File] FileTypeExtension : jpg
[File] MIMEType : image/jpeg
[File] ExifByteOrder : Big-endian (Motorola, MM)
[File] ImageWidth : 4000
[File] ImageHeight : 3000
[File] EncodingProcess : Baseline DCT, Huffman coding
[File] BitsPerSample : 8
[File] ColorComponents : 3
[File] YCbCrSubSampling : YCbCr4:2:0 (2 2)
[IFD0] Artist : Richard Pillay
[IFD0] Make : samsung
[IFD0] ImageHeight : 3000
[IFD0] Orientation : Rotate 90 CW
[IFD0] ModifyDate : 2023:11:13 11:20:33
[IFD0] YResolution : 72
[IFD0] XResolution : 72
[IFD0] ImageWidth : 4000
[IFD0] Model : SM-G998B
[IFD0] Software : G998BXXS9EWJO
[IFD0] YCbCrPositioning : Centered
[IFD0] ResolutionUnit : inches
[GPS] GPSLatitude :
[GPS] GPSAltitude : undef
[GPS] GPSLatitudeRef : Unknown ()
[GPS] GPSAltitudeRef : Above Sea Level
[GPS] GPSProcessingMethod :
[GPS] GPSLongitudeRef : Unknown ()
[GPS] GPSTimeStamp : 00:00:00
[GPS] GPSLongitude :
[GPS] GPSDateStamp :
[GPS] GPSImgDirection : undef
[GPS] GPSImgDirectionRef : Unknown ()
[ExifIFD] ExifVersion : 0220
[ExifIFD] ApertureValue : 1.8
[ExifIFD] ExposureProgram : Program AE
[ExifIFD] UserComment : Yaw:120.81007,Pitch:-20.26522050904004,Roll:86.95352588209022
[ExifIFD] ColorSpace : sRGB
[ExifIFD] ImageUniqueID : XA8XLNF00SM XA8XLNF00SM.
[ExifIFD] ISO : 50
[ExifIFD] ComponentsConfiguration : Y, Cb, Cr, -
[ExifIFD] MaxApertureValue : 1.9
[ExifIFD] FocalLengthIn35mmFormat : 24 mm
[ExifIFD] SubSecTimeDigitized : 679
[ExifIFD] DateTimeOriginal : 2023:11:13 11:20:33
[ExifIFD] SubSecTimeOriginal : 679
[ExifIFD] WhiteBalance : Auto
[ExifIFD] CreateDate : 2023:11:13 11:20:33
[ExifIFD] MeteringMode : Center-weighted average
[ExifIFD] FocalLength : 6.7 mm
[ExifIFD] ExposureTime : 1/50
[ExifIFD] Flash : Unknown (0x2)
[ExifIFD] SceneCaptureType : Standard
[ExifIFD] SubSecTime : 679
[ExifIFD] FNumber : 1.8
[ExifIFD] LightSource : Unknown
[Composite] Aperture : 1.8
[Composite] ImageSize : 4000x3000
[Composite] Megapixels : 12.0
[Composite] ScaleFactor35efl : 3.6
[Composite] ShutterSpeed : 1/50
[Composite] SubSecCreateDate : 2023:11:13 11:20:33.679
[Composite] SubSecDateTimeOriginal : 2023:11:13 11:20:33.679
[Composite] SubSecModifyDate : 2023:11:13 11:20:33.679
[Composite] GPSDateTime : 00:00:00Z
[Composite] GPSLatitude :
[Composite] GPSLongitude :
[Composite] CircleOfConfusion : 0.008 mm
[Composite] FOV : 73.7 deg
[Composite] FocalLength35efl : 6.7 mm (35 mm equivalent: 24.0 mm)
[Composite] HyperfocalDistance : 2.97 m
[Composite] LightValue : 8.3