Hryol My understanding is that the gesture UI code and the three-button UI are maintained by Google and unmodified by GrapheneOS from the AOSP base code. If you have access to a friend's device running Google's OS, updated to A15 QPR2, you might be able to see if it's broken there too (I suspect so, because Google doesn't appear to be very committed to the three-button UI).
My hunch is that zero GrapheneOS developer hours are being spent looking into this and likely none will -- GrapheneOS is focused on security and privacy, which are not apparently affected by this minor UI regression. Sometimes Google fixes something like this themselves in AOSP, in which case GrapheneOS will pick up the fix.
Please note that I do not speak for the GrapheneOS project.