Preventing unwanted change propagations?

This is not a bug report or a complaint, more of a ‘best practices’ or ‘how do I do this?’ question.

I use Nextcloud to keep my photo collection on my Android phone sync’d with my home server. Some of the photos originate on the phone, most don’t. Sometimes Nextcloud suddenly decides it needs to upload gigabytes of photos from my phone to my server. I don’t know why. This in turn triggers gigabytes of downloads on my tablet, my laptop - wherever other copies of the photos are sync’d.

I suppose there is some app that is modifying my photos (perhaps embedding a thumbnail, perhaps ‘helpfully’ adding some EXIF tag?) If so, the core problem is really with the app doing that. (I don’t suppose the Nextcloud app itself could be tweaking the files?)

Short of that, are there any options in Nextcloud that could be helpful? Perhaps previewing updates before attempting them? Or comparing files only by name?