[deleted] Though it is difficult to be sure, this sounds like it might be a hardware failure -- in particular, the flash storage.
If that is the case, it is possible that a factory reset (though disruptive, since it would wipe all of your data), or a complete reflashing, might detour around the bad part of the flash.
But if it's still under warranty a replacement might make more sense.
[deleted] I searched for possible fixes and found out that this is a common pixel bug faced by users with multiple profiles on stock rom.
I don't think this is a common problem for GrapheneOS users.
[deleted] Should I wait for Android 15 update and hope that System UI app is updated and I’ll get access again?
Personally I would pursue things like factory reset, re-flashing GrapheneOS, flashing back to Google's stock OS, or a warranty replacement rather than waiting for A15.
[deleted] Can I flash systemui app? It worked for someone on stock rom.
I don't think so.... and it sounds as if maybe more things than just that one app are bad.
[deleted] Should I wait for a bug fix, in case it is easy and doable?
I am unaware of a bug in GrapheneOS with this shape (that doesn't mean there isn't one, of course).
Others may chime in with other thoughts.