I compared recent old and new iPadOS movies shot with the same iPad and it seems that the culprit is this new tag in iPadOS 18:
[QuickTime] Full-frame-rate-playback-intent : 1
I could remove or add that tag with the help of this example.config with exiftool (exiftool does not yet have that tag included so it must be added as a user defined custom tag via a config file. You can put that text file named as "example.config" to the same folder as the images so it takes effect in the following commands):
%Image::ExifTool::UserDefined = (
'Image::ExifTool::QuickTime::Keys' => {
'full-frame-rate-playback-intent' => { },
},
);
Remove the tag:
exiftool -config example.config -m -P -overwrite_original '-QuickTime:Full-frame-rate-playback-intent=' movie.mov
Add the tag:
exiftool -config example.config -m -P -overwrite_original '-QuickTime:Full-frame-rate-playback-intent=1' movie.mov
I tested this via Windows 10 VMware Fusion 13.6.1 virtual machine on macOS 15 Sequoia and at least there the thumbnails could be seen after that Full-frame-rate-playback-intent tag was removed in H.264 movies. My Windows 10 does not have H.265 add-ons so those movies remained without thumbnails. Can you verify that also H.265 behaves the same?
I do not yet know the purpose of that tag so I would not lightly remove it. Instead, I'd wait for a Windows update to fix this.