The only folder Explorer produces PDF thumbs for me is in OneDrive - it won't do it in a normal folder.
The issue you've seen seems to be a bug with the thumbnail generation in OneDrive folders when a size greater than 256 is requested. It doesn't seem specific to PDFs though, an image file will do it as well if the file is in offline storage.
Many thanks, Jon. For all the good it will do. I will report it to Microsoft.
Microsoft seem to have produced a partial fix to this problem via a recent Windows 10 update (possibly KB4051963). After applying that patch, now when I increase the thumb size to larger than 256, the thumbs remain visible at 256 in size but they are surrounded by the space that the larger thumbnails would occupy.
It is a good step forward, for at least I can now have my image thumbnails at the size I wish safe in the knowledge I will not have to resize and recreate the OneDrive PDF thumbs.
Are you sure it was that and not the Opus update? We made a change on our side to only request max 256x256 thumbs for remote OneDrive files.
I am sure it is the work you have done. Microsoft does not respond that quickly. (In fact, I was gobsmacked when I noticed the fix) I happened to update tp the latest Opus and receive that Microsoft update on the same day.
The way the PDFs from OneDrive appear in Opus at bigger than 256 also bears this out.
Anyway thanks very much, it may seem like only a small thing, but it could really waste time.