Do the filenames have any accented characters (non-ASCII) characters in them?
They did not appear to have.
I seem to have solved this by;
Copied filename from combined raw+jpg pair in contact sheet
Pasted to BBEdit and added a "-A" suffix
Pasted 'new' filename back into PM+
Image now shows correctly as pair in both folder contact sheet and save search.
I've now removed the suffix and its still good.
Who knows but thanks for the tip Kirk I seem to have cleaned up something, somewhere.