Facebook Videos Don't "wrap"

lately i have ever-increasing difficulty getting videos to insert into FB messenger. they appear to upload just fine -- and will replay for me locally during the session -- but never quite trigger the "check" mark.

they are gone upon any refresh.

what gives? i am mostly just using videos DLed from youtube, generally via 4K. i have tried changing them from MKV to MP4 or AVI as a workaround, but still no go.

is 4K doing something weird to the files to begin with? i don't recall any of these probs when the vids i was uploading came via YTD. even now, i can upload vids np if they're ones i've had kicking around my PC from years past. but anything recent / new plucked off of youtube is hopeless!