P: Custom Order lost with slight accidental movement of thumbnail in filmstrip

Do you use a mouse or Wacom (or similar) ?

Yes. Though I just moved a few photos around in the small folder, and the problem didn't rise, so I tend to think that the number of elements is key.

Had you ever reordered the photos in that folder any time before?

Currently 373 photos (about half of which are PSDs).  And no, as soon as I switch from File Name order to Custom order the problems begin. But the number of photos may be relevant, as I just tried with a much smaller folder (12 images) and the order was maintained...

Alessandro,

How many photos are in the folder? Have you done many rearrangements of the custom order of the photos in the folder (such as dozens of insertions, moves, and edits in the custom order)?

Not here.  If I have Stack with Original ON the PSD file and Original are still in the correct custom sort order.  

That's exactly the problem: I have "Stack with original" ON, the PSD file is correctly stacked with the original file, but both files are moved to the end of the strip.

Starting with Custom order an Edit In Photoshop will return the photo next to the original, not at the end.  This is with "Stack with Original" OFF.  Lr9.3 and macOS 10.15.6

Well, it's different in that it only happens when the order is set to Custom: if I use File Name or Capture Time, the order is preserved.

No, this is the same old problem that we have been experiencing for years and years now.  Sort order is lost many times a day when working in LR, whether working just in LR or working back and forth in LR.  The only answer I can see is for Adobe to put a lock on the sort order.  (as they have not been able to find any other solution to this time excessive issue)