Ok, i found out the Preview generator app on the appstore, installed it and ran the occ job and looks like the thumbnails previews are being generated for the .jpeg or .jpg but not for .heic on the ios.
My question now is if there is a job that can update the thumbnails on the ios app??
Both the website as well as the apps read from the file cache, where the thumbnails are being stored as well. As for the .heic ones, I just went through the changelogs and could not see anything regarding that, so I guess they are still working on it and it will appear in one of the next releases.
There is no seperate job to generate thumbnails in the iOS app, since the apps are also using the cache from the server. It depends on your connection to the server, how fast the thumbnails appear.
I can confirm: using HEIC images for a while now the Nextcloud iOS app is not showing any thumbnail AT ALL in “media” tab for images until I manually open them, switch back and - voila - there´s a thumbnail.
So I guess it´s the app being not able to generate HEIC previews?