I can also report high % of stales / rejected (>10%) with RX 6600XT with Phoenix 5.7b
With NBMiner 39.0 only about 0.05% stales/rejected.
OS: Windows 10 Version 2009, 10.0.19043
Driver: non-whql-radeon-software-adrenalin-2020-21.8.1-win10-64bit-aug10
But imo the biggest problem is that after about 1-3 days mining, 6600 XT's start doing 100% incorrect shares and because of that, they are not anymore regonized by a pool and you lose money. (after restarting miner all works good again, but its not nice to wake up in the morning and see that your 6600 Xt or more of them has been hashing incorrect shares whole night)
I have two different rigs with RX 6600 XT gpus: both of them are mixed with Nvidia cards, and only those RX 6600 XT:s starts doing 100% incorrect shares after that 1-3 days of mining. This has happened never before any of my other GPU's and i have been mining over one year with Phoenixminer which has been my absolute favourite... until now.
Phoenixminer with Radeon 6600 XT is incredibly unstable at this moment. I'm really hoping to see somekind of fix to this situation.
Phoenix miner has always had a lot of problems with the 6000 series (I am owner of 6800 Xt) and even now with this new release 6600 XT it is the same.They fixed the problems in the upcoming versions of their miner and I think you only have to be patient until the next release which will most probably contain a fix to this,until then you can switch miner as the other option to stop getting incorrect shares.
I have a 6900xt and never had an issue running on the 5.6d version or the 5.7b but running the older 21.5.2 drivers