[I've searched through Phabricator and can't find anything like this, but this is at tricky search. The closest I found is T208868, which is clearly a very different bug. If this is already fixed, great, but otherwise...]
In a two-week-old MediaWiki 1.35 installed under Ubuntu 20.04, with Firefox as the client:
I originally thought this might be my preferences somehow not getting saved, because I would uncheck "Group changes by page in recent changes and watchlist" and would find it magically checked again after I tested whether it had affected RC. But I could load other pages, and I could also either hop around in the Preferences tab, or reload the page, or load it fresh in a separate browser tab, and the unchecked setting would stay unchecked, -until- I reloaded Recent Changes---whereupon Group was magically checked again as soon as I reloaded the Preferences page.
And other preferences -do- get saved normally, so that rules out (for example) some weird 302 POST problem because I'm using shortened URLs---and, having figured out the workaround where I turn off the JS interface and my grouping preference then get saved, it's clear that prefs are really being saved.
If someone needs me to reproduce this, I have a copy of the database from that time and could if necessary reload it. I would rather -not- have to try to reproduce this in 1.36; I'm running 1.35 because it's LTS and I don't want to be on an upgrade treadmill with a non-LTS release. (It would be a lot of work to install 1.36 in parallel.)
Note that the client side of this was Ubuntu 14.04 running an old (66.0.3) version of Firefox. It's got NoScript running but I've told it to trust scripts on my own wiki. I could attempt to reproduce this in 20.04 with the current version of Firefox without much trouble.