>  > It does not happen with Emacs 23 to 25, and happens with Emacs 26 to 28.
 >
 > That's interesting.  I'll have to take a look.

OK.  It was me who introduced that bug in Emacs 26.  Many thanks for
spotting it.

Can both of you confirm that reverting that change fixes the behavior?
Patch attached.

Thanks, martin