On Sat Nov 14 2020 Teemu Likonen wrote:
> This can be fixed by locally let-binding fill-prefix every time in
> the relevant filling function. Patch for that is attached.

Your patch implies that the buffer-local value of fill-prefix is not
what it should be (say, if any other command wants to use it).

How about the rather different patch attached below, partly inspired
by the related comment

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21764#15

which had somehow escaped my attention.  The issues mentioned in
this comment should likewise be fixed by the attached patch.