Sync with HEIC files doesn't work for me either. This has been reported before:
https://piwigo.org/forum/viewtopic.php? … 42#p188942

It seems that processing is different for uploading vs. sync. During an upload, you can see the ImageMagick "convert" command being executed in the log:

[2024-09-11 20:00:44]   [INFO]  upload_file_heic, exec = convert -sampling-factor 4:2:0 -quality 85 -interlace JPEG -colorspace sRGB -auto-orient +repage -resize "2484x1863>" "/mnt/piwigo/_piwigo-sites/piwigo-beta/upload/2024/09/11/20240911200044-a4f8fa6e.heic" "././upload/2024/09/11/pwg_representative/20240911200044-a4f8fa6e.jpg" 2>&1

But this doesn't seem to happen during a sync, so the JPG thumbnails for the HEIC image are not created in the album and you end up with the "unknown" question mark thumbnail instead.

Also keep in mind if you use "Add Photos" you're not actually adding that photo to the album/folder you linked under galleries, it's going to end up in the virtual album folder structure under uploads.