I can confirm this. My large video also cant be synchronised after uploading via ftp/smb

I have been able to successfully import MP4 files using the FTP+synchronize feature of Piwigo 15.5.  I did not use FTP to upload the files as the MP4 files were on the same file system as Piwigo "galleries" and I could just use a "cp" command.

My Piwigo server is running on an Ubuntu 22 system.  Per the Piwigo documentation I created a directory under "galleries" taking care to ensure that the new directory "galleries/xfer" was owned by "www-data" as this is the ID that the apache2 server is running as.  File and directory ownership is key to synchronization.

I copied the MP4 files into "galleries/xfer" and made sure that the files are also owned by "www-data".  I then went to "Administration / Tools / Synchronize" on the Piwigo GUI, making sure that "directories + files" was checked, "Simulation" unchecked, and then hit the "synchronize" button.

At this point I am able to see the "xfer" album and all the MP4 videos indicating a successful synchronization.

The problem is that their are no thumbnails associated with the MP4 files.  If you upload small MP4 files using the Piwigo GUI, the thumbnails are automatically created and all is well.  Using FTP one must create a "pwg_representative" directory within "xfer" and copy a "jpg" file with the same same.  Hence you would have:

    galleries/xfer/test.mp4
    galleries/xfer/pwg_representative/test.jpg

Make sure that www-data owns all the files and directories before hitting the "synchronize" button again.

Since the problem you are seeing is size dependent at what file size does the import fail?

I don't know if the video playback plugin matters on import but I am using VideoJS.

So how big  does the video file need to be before it becomes invisible to Piwigo?

When you FTP the <2GB file to the /galleries/SomeFolderName which is then seen by Piwigo, do you also see a thumb nail associated with the file?  I noticed that thumbnails are NOT being generated during the synchronization process and if you are seeing thumbnails then Piwigo is doing some process, which may be subject to a size limitation, that is missing from my synchronization process.

Likely some code (be it a plugin, Piwigo, or anything else) in the chain uses an int32 variable to process file size, that would exactly hit the 2147483648 bytes (2GB) maximum limit.