I moved here in mid 2021 myself.
I have done a quick search but not sure what is definitive or not. I received the communication from the landlord with the below. Seems quite cheeky to raise up so much.
woooo, that's crazy. If I understand well teuerungsausgleich is inflation and they have not increased rent since 1993.
What does your rentral contract say with regards to what the rent is indexed to?
I think it can be indexed to one index only and not two? Your increase mentions both the teuerung ausglech and the hypothek index. Someone more knowledgeable will come along with this info