Now my real question is... Why wouldn't Slushpool err Braiins work with the S21+?
Slushpool might be old and no updates anymore. 4 years ago, Slushpool and all its content were moved to Braiins. If the Braiins pool works for you, then better use Braiins pool since it's also owned by Slushpool. There's no problem with
I do not think that there is anything left of Slush, it became Braiins everything (pool and firmware), so the problem is indeed with Braiins' pool, i would be very surprised if this happened to OP while using the stock firmware, that would be a huge fuckery by Braiins.