Looking at the sim spell data for Arcane T20, the triggers are wrong in the default version. The 2pc effect was set to trigger with 4 pieces and the 4pc effect was set to trigger with 2 pieces. Also the spell ID for the 4pc was pointing to the wrong ID. Not sure if this will ultimately effect the gear rankings as right now the default seems to end up ignoring t20 entirely when you run a BiB or BiS gear sim.
That is one of those sets that they swapped at the last minute… I’ll get it fixed up.
Sounds great. 
It seems like with that fix the BiS finder tends to recommend t20 2pc, but not the 4pc. Happens across all gear levels I checked.