I am having exactly the same issue as here display-settings-not-working-as-exspected. It says in the thread, that it was fixed in 1.18 version, but I am having this now on 3.0.1-a14.
So in short, when selecting option for 90hz it changes the refrez rate properly. However, when closing screen and opening again, the refrez rate is not 90hz anymore, even though it shows that still in the settings. Wierdest part about this is the fact, that opening camera app puts it back to 90hz, until I close the screen again.
Issue was sadly reintroduced with a later update, but luckily there is new activity in the bugtracker aka it was acknowledged and assigned to a project/developer, which means somebody actively works on it and will hopefully fix 90hz in the near future.
Alright, good to know this is being worked on. I tested some more and the same issue is happening also with calyx OS, which I tried today. So the issue might not be in the /e/os itself?
Maybe it’s broken upstream, so maybe we’ll have the technical wonder of downstream Murena OS fixing an issue and reporting it upstream and fixing it for everybody. Fingers crossed.