They were never that objective in any other debate Trump has been in as they were against Biden. The earlier debate by far, neither were even officially the nominee yet. They were at least determined to be objective for the first time when Trump was in one. Even Fox News with Megyn Kelly and Chris Wallace were hard on Trump, clearly biased against him.
The biggest tell though was the reaction across all media after the debate. They went ridiculously hard against Biden , all saying the same things "I'm getting calls from my sources and the Democrats are worrying" etc.
Look, it's impossible not to dissect this stuff considering what we have seen over the last 8 years.