Yes, I can confirm this. The High profile is broken. Fast and Balanced still work, but they don’t use the GPU and are painfully slow.
Interestingly, unmixing just the vocals does trigger the GPU, for whatever reason, but the result is just an empty track, so that’s useless too.
I’m rolling back to SL11 until this is properly tested and fixed. Not exactly thrilled to be $72 down for a downgrade.