It's not the rng... all i know is that it is lowered. That is all.
It is not lowered. But even from the start till now the 'rate up' event is negligible. There is Lot of people that didnt get Vincent when Vincent rate up, but he get Lot of Theresa instead. Some people got many cybella when there is KAI rate up event, and he dont get any KAI. The only rate up that I personally feel it 'up' is winry. Maybe because she is 4*.
We dont really know about this rate up. It could be 1%, 10%, 50%, 100% rate up. Lets say its 100%, we still dont know it is 100% rate up from what? It could be 100% rate up from 1%, so the final is only 2% chance to get Miu. It is hard to know how the calculation work, it can be direct rate up which mean for every Card you draw u have rate up for Miu, or it could be rate up to get Miu from 5* capable hero draw which mean u should get 5* capable hero first ini the calculation then you Will get rate up probability to get Miu, while another 5* capable hero is having normal rate.
All in all in my opinion the 'rate up' is ambiguous. But this ambiguous rate up is normal for many games that I play. But sure I play another game that give detailed rate up event like ' at this week you have a 5% chance to get XXX heroes from each premium draw '.