Wallace's bias was most evident in two ways:
As a debate moderator, Wallace's big problem is that he treats it like a Sunday show interview.
Wallace winds up a big question with a preamble using prior quotes and recent event references, and then tries to string all of that into a "gotcha" question.
The problem is that in a debate, that takes up too much time and the debaters are anxious to get going. As soon as Wallace says something incomplete or out of context, President Trump jumped in to dispute the question.
It's Wallace's own fault for not setting up a simple question and letting the candidates take turns answering. When Wallace asks Trump about "$750 per year income tax," how can Biden fairly answer that question other than to use it to attack Trump?
If Wallace asked "What is your opinion on releasing personal information like tax returns and medical records?" to each of them, that would be a simple and fair question. But he doesn't do that.
-PJ
“When Wallace asks Trump about “$750 per year income tax,”
Which is starting with a conclusion: that it was a true statement which it was not.