We believe that this is still locked due to the R7 - R12 Levels being locked on the current game client. The developers only opened the Proving Grounds R-0-6 due to the Fire Lord Monster event and it does not spawn in Proving Grounds R7 - R12
Well i dont really care bout the spawn but im no longer able to do PG. So ppl with r7+ are just screwed ?
We will have to look into this once we have the R7 - R12 update, but it looks like it was still locked from the previous Rebirth 6 update we will have to double check.