Thanks u/magthe0 and u/tarsius_, that rang a bell. Unfortunately, that doesn't seem to be the problem. The refspecs have been added to the .git/config files of the clones (I usually keep my working copies around and rarely clone from scratch).
As a test, I just (freshly) cloned one of my projects in a subdirectory /tmp and indeed, after running M-x forge-add-pullreq-refspec, the following was added to the .git/config file:
fetch = +refs/merge-requests/*/head:refs/pullreqs/*
for remote origin. This looks fine (and such a line is present in the 'regular' clones of my projects as well). Next, I ran M-x magit-fetch-all, as well as N f f to tell Forge to fetch all topics. The Magit status window shows the "Pull requests" and "Issues" sections and both are populated (with proper !123 type coding for the merge requests). However, when I open the magit log screen I am back where I was before: I don't see the MR numbers as refspecs there (I have the -d Show refnames (--decorate) option turned on.
Side note: I'm not a master of the Git plumbing commands, but git show-ref doesn't seem to list all MRs. For this test project I have 10 MRs (all merged or closed), but only 9 and 10 show up:
$ git show-ref
f0aa1b61be5ea74c47c473a716f209c698a1a006 refs/heads/master
68e5d70e05fbdc6d47e8cb6da903dcd89d4432ea refs/pullreqs/10
886d7067aa3e609e1b11560a949182dadc57401b refs/pullreqs/9
f0aa1b61be5ea74c47c473a716f209c698a1a006 refs/remotes/origin/HEAD
f0aa1b61be5ea74c47c473a716f209c698a1a006 refs/remotes/origin/master
However, in the Magit status view, I do see the last 5 and M-x forge-list-pullreqs lists them all.
Thanks a lot for digging that up, u/tarsius_! In the example repo I mentioned above, I now see MRs
!9and!10in the Magit log view (well, without the!, but that is fine).The fact that I don't see older MRs probably has something to do with the corresponding refs not having been downloaded. I'll see if I can figure out how to do that.