Found another trick I'd not known of until now. Import the MP4 via VLC File Sharing in iTunes. In the FIles app I see the MP4 in the VLC folder. Tap it there and it plays in VLC - but still not shown on the Video page.
Trick: Go into settings and toggle Disable automatic media grouping. Back to the video page and presto, there's your recently uploaded file.
Just something to try. I still see inconsistent behavior though.
Still the secret seems to be to make sure VLC is not running in memory when adding files via iTunes - especially if uploading more than 1 at a time. Because I just uploaded 2 and the 2nd one was not recognized/added to the grouped folder. Toggle grouping OFF, now it's there - though with a blank thumbnail. BUT, toggle grouping back ON, it's GONE.