Sidebar preview fails in Win10 - sticks on first generated preview, will not change

My sidebar previews worked fine until I installed KLite Code Pack. Now, I get a preview in the right-hand sidebar for the first video I click on, and thereafter it's stuck showing that, no matter what other video I click on.

My registry entries look like this:

HKEY_CLASSES_ROOT\.mp4\ShellEx\{8895b1c6-b41f-4c1c-a562-0d564250836f}
(Default) = {031EE060-67BC-460d-8847-E4A7C5E45A27}
HKEY_CLASSES_ROOT\.mp4\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}
(Default) = {c5aec3ec-e812-4677-a9a7-4fee1f9aa000}
HKEY_CLASSES_ROOT\.mp4\ShellEx\{E357FCCD-A995-4576-B01F-234630154E96}
(Default) = {c5aec3ec-e812-4677-a9a7-4fee1f9aa000}

This is pretty typical. E.g., the M4V entry is identical, other than ...\.m4v\... instead of ...\.mp4\....

I was originally using VLC (and miss it already), but have been trying KLite's bundled MPC-HC 64-bit player.  I have no interest in using Microsoft's WMP.

I'm not having any issues with generated thumbnail icons.

This is a bug in the preview plugin that Explorer uses. That is an old component in Windows.

But it is interesting that it worked before.

It uses the standard reg keys for the preview. So that isn't the problem.

Try this:
Codec Tweak Tool > Preferred decoders > Reset (to Windows defaults)

That didn't fix it, but I'll need to restart at some point, and maybe it'll affect it then.  I'm not sure the playable video preview widget I'm seeing in the sidebar preview pane is even the one I had before.  But, yeah, it worked fine for me before, both on this real Win10 box and in my Win10 VM under macOS.  In fact, my main way of knowing whether one of my flakey media-downloader apps worked on everything or not was that all the videos that downloaded properly were previewable while the ones that got aborted were not.  Now I just get one preview, and it never changes (in that File Explorer window, even if I go to another directory).

So, I'm guessing some component of KLite has replaced whatever Windows was calling out to before to render these previews. Not only codec-wise, which you just had me turn off, but also UI-wise.  My experience with KLite is just about nil, though.  Ideally, it would use what it was using before, but with modern codecs, and this wouldn't have any effect on what application I say to manually open a .mp4 with (and I may go back to VLC for everything that it supports, since I just know all its keboard shortcuts better, and I think I actually get smoother performance from it). I'll only need KLite for stuff VLC can't handle.

The video preview uses the "WMP Rich Preview Handler".
There are no changes in the UI or to the preview handler.

It is a known issue on Windows 10. I have been able to reproduce it even on a fully clean install of Windows. Not only does it get stuck, it sometimes even completely hangs Explorer. On Windows 7 and 8.1 it doesn't have those problems.

If you are just interested in checking if the file is playable, then thumbnail view should be enough.

Removing HKEY_CLASSES_ROOT\.mp4\ShellEx\{8895b1c6-b41f-4c1c-a562-0d564250836f}
Disables the video preview. Then you get a thumbnail instead.