Video thumbnails broken

Our video thumbnails either do not appear, or sometimes show up for a second while the page loads and then disappear. A few of them show up persistently, but sometimes disappear for bit, until the page is refreshed.

Solved this by

mp4box -brand mp42:1 -add input.mp4 -new output.mp4

The ffmpeg output of the working video is:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'room1na/output-ben_room1_foot.mp4':
    major_brand     : mp42
    minor_version   : 1
    compatible_brands: isommp42
    creation_time   : 2023-04-14T20:23:48.000000Z
  Duration: 00:01:55.64, start: 0.000000, bitrate: 1580 kb/s
  Stream #0:0[0x1](eng): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709, progressive), 1280x720 [SAR 1:1 DAR 16:9], 1580 kb/s, 15 fps, 15 tbr, 1000k tbn (default)
      creation_time   : 2023-03-09T21:46:28.000000Z
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]