Well of Time BUG stage 12-6

When I click stage 12-6 it said "Stage is locked", I already finished stage 12-5 , it used to be working, I even use it to test my unit skills before, now I've been stuck here for a weeks. fix this please, I cannot progress my immortal to tier 20, sorry if my English is bad, btw I'm playing on STEAM/PC that's all thanks Merry Christmas everyone!.

It said its "Cleared" when I didn't.