AMD drivers 21.3.1 (Windows) and 20.50 (Linux) are not supported by PhoenixMiner 5.5c. Do not upgrade to them, or you will get errors, lower hashrate, high stale shares, etc. Support for these drivers will be added in the next release of PhoenixMiner.
I'm using AMD drivers 21.3.1 (Windows) in 5.5c with my RX 6800 getting 63 MH/s, no errors, 2% stales (1 in 55 valid shares), so it's very weird.
I tried AMD drivers 20.11.2 (WHQL) and were very buggy, 35 MH/s or something.