2.36.0: enormous numbers of loose objects after enabling partial clone filter: why? how to clean up?

So I turned on promisor remotes for my local Chromium tree last week: the relevant remote now has partialclonefilter = blob:limit=10m. I wanted to save a bit of space (well, OK, I was hoping for a lot).

This turned out to be... a bad idea. The first fetch after that exploded some 45GiB of loose objects (nearly six million of them). I spotted an ancient historical config option gc.pruneExpire=never and removed it: that would explain the worst of the accumulation, if the files weren't all dated at around the time of my first pull after turning on partial clone.

A git repack -ad, git prune --expire=now, and similar expiry of all the reflogs has got them down to a mere 112957 objects, 597304 kilobytes (!) but I can't get the count lower than that, despite deleting every remote (to get rid of the remote-tracking branches) and doing a full repack (non-aggressive), prune, and a git prune-packed (which had no effect at all). I've done not a great deal of work in this tree and no rebases: there should be hardly any legitimate loose objects, I'd have thought.

Indeed, git fsck --verbose --name-objects shows that all these objects are referenced, mostly in a few fairly recent branches. They also clearly reside upstream, in the remote-tracking branches. So... why is git repack refusing to pack them? Is there any way to find out? It's fairly opaque in its decisions, which usually I don't worry about, but this space bloat is kind of ridiculous.

(There are no alternates or anything like that pointing at this repo, so it's not that.)