Thank you for describing my problem so thoroughly, I have the exact same problem and as a new owner of a Poco F2 Pro (rom: V12.0.3.0.QJKEUXM) this is a real deal breaker.

I am on a router of Synology (RT1900ac) and tried all sorts of remedies on router side:
-update to latest firmware
-split up the 2.4/5ghz signal (happens on both signals)
-use WPA2 Personal / WPA+WPA2 Personal
-Automatic channel/fixed channel
-5ghz in DFS channels and non dfs channels
-Enabling/disabling U-APSD (power saving)

All of this has been tested with the different settings on the phone:
-Wifi select best networks automatically on/off
-Developer access "Limit Wireless scanning" on/off
-Disable Wifi access for Google location services

Checking the logs of synology Wifi I see a lot of connection errors for the Poco device that I do not see for other devices (such as the xiaomi a1 I have): 1, 3, 6, 7, 15, 16
If you look at that table ... ation-Reason-Codes- then you will notice that 16 is the "Group Key Handshake timeout" and if I read correctly that is the code that Xiaomi used to restart the wifi modem on the hotfix .5 release for red mi 9 (not the actual fix in the .11 release).

It did not happen when connected to a hot spot so it must be a combination of factors. I'm going to check with different routers.