I have literary thousands of .mp4 videos on my PC. Until 13.5.3 each video regardless of the video player associated would show a picture preview of the .mp4 video.
This no longer happens regardless of the video player associated with my .mp4 videos.
Do the previews come back if you revert to 13.5?
FWIW, this has been happening to me since...forever?
I didn't reported it because was grateful enough that v13 does provides all media-info 
I tried to narrow down the issue, first thought because they're HDR videos, but some do work, then thought by using the last version of mkvmerge, but also found some that does also work. 
Presumably it's the same in File Explorer as well? If so, the required codecs/splitters aren't installed for Windows to thumbnail the video.
Ah right, silly me, yeah it's the same in Explorer sorry (didn't even thought on checking it, haven't used it in ages :))
If it does matter, those videos does play fine in the DOpus's Viewer, it's just the thumbnails that doesn't get created, the only system-wide codecs i do have are HEVC & VP9 which came pre-installed (w11 for Workstations), and i don't want any other bloatware since i mainly use mpv which can play anything.
Well, perhaps you guys can consider some fallback logic here to create the thumbnails using your movie.dll or such when the system cannot do it, dunno, no big deal anyway
Something has probably broken the filetypes in the registry if playback works but thumbnails don’t. (Assuming this happens in all folders, including the desktop.)
Changing the default movie player in Windows to the stock Microsoft one may fix it.
er.. then might be because i did uninstalled the stock player.. and a bunch of other apps forcefully through PowerShell, as i said don't like bloatware 
dunno, It's interesting because afaics the thumbnails only do fail for 10-bit videos (?)
Failing:
Stream #0:0: Video: vp9 (Profile 3), yuv444p10le(tv, progressive), 1536x864
Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, bt2020nc/bt2020/smpte2084), 1920x960
Stream #0:0: Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(tv, bt2020nc/bt2020/smpte2084), 3832x1600
Ok:
Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv), 360x360
Stream #0:0: Video: hevc (Main), yuv420p(tv, bt709), 1920x1080
Stream #0:0: Video: hevc (Main), yuv420p(tv, bt709/bt709/unknown), 1920x804
Getting nuts... this one is 8-bit but also failing, it's 8K tho.
Stream #0:0: Video: hevc (Main) (hvc1 / 0x31637668), yuvj420p(pc, bt709), 8192x4096 [SAR 1:1 DAR 2:1], 138356 kb/s
(Source: NASA Image and Video Library )
Well, meh 
