mp4 thumbnail changes will not keep- reverts back to application icon

I have been updating my mp4 thumbnail files for years via copy and paste within the get info dialogue menu for each mp4 file. recently my MacBook will not display the thumbnails of some mp4 files and uses the standard video player icon. I notice that when trying to update the icon thumbnail that the finder window will flash and the mp4 icon does change to my desired image, but then it immediately reverts back to the standard icon. if I dismount and remount the usb drive the changes do not show up ( sometimes they do show up but then go back to standard icons immediately)

mp4s are stored on a usb thumb drive formatted as exfat

MacBook Air m1- ventura 13.1 (22C65)

icon preview checked on finder folder get info