This is very sad! I hope that in future Newtek will find a way to correct this. I think it is unacceptable to can not rename your outputs. In whole world in video production society Mix/PGM is use for primary output name and other outputs are called auxiliary outputs.That is what I heard from tech support :
This is intended. Unfortunately there are a lot of complications to implement that so it does not break current workflows or cause NDI sources to be lost (since it matches the output name).
I think it's related to this line in one of the latest release notes. Crazy this is mentioned as a bug fix, while actually it's a feature removal.
Bug Fixes : Corrected an issue where renaming Mix outputs would cause local Mix input sources to not show.
This is actually extremely production critical, especially when you have multiple tricasters on the same network. Then this becomes very confusing, here improvements must be made not the opposite that has been done in this case