This should be MIUI issue, i had same issue on MIUI 12 but when i flahed a pixel rom i was with highest hz without issue anymore.Today my Poco F3 snapped to 60hz, the first time ever. In settings was 120hz, after setting to 60 and back to 120 everything seems to be okay again.
Xiaomi eu team just doing a porting of china based MIUI, they dont modify or fix MIUI issue.