Hi Kirk, thanks for your reply. I did what I usually do: open the Trash folder and drag the raw file + sidecar onto the PM contact sheet. It (almost) always appears immediately which means I have the settings that normally work. I don't think this is different from moving or duplicating images between folders using PM contact sheets (which automatically moves the sidecar files with the image files).
I do not believe there were any filters. As I said, restarting PM fixed the issue, and doing a Rescan and Refresh did not fix it, although I can't be sure that doing this in combo before a restart made a difference (I doubt it). I have had the experience where I drag over a file and the Sort setting changes from Filename or CaptureTime to "Arrangement". In that case I have to change it back to bring the files back in order. I checked for this in my trouble shooting.
It has not happened again since this one time. As I said, assuming this is not something that becomes recurrent, I just wanted to leave a solution if it happens to someone else.