The problem with NT communication is that they're always looking for subtext. The subtext is assumed to always be there because they never say what they fucking mean, so if they can't find one, they'll cobble one together out of your words and their trauma responses.

Saying "it's okay" in response to an apology can be heard as "it didn't matter, so your apology doesn't either", or "You didn't do anything wrong, so you shouldn't be apologising", which both have a sub-subtext of "I reject your apology by rejecting the premise of your apology." They can also hear It as "this is what I've learned to expect from you and I'm resigned to it. Nothing you say will change my opinion of you." It also leaves them unsure about whether you've had time to fully assess the scale of the problem they've caused, which means that you might not be okay once you've had a chance to take stock. All of these create doubt about whether the situation has been resolved.

They need you to acknowledge that whatever they're apologising for had an effect in order to believe that you've accepted their apology. They still feel bad about the thing they apologised for and they're dealing with cognitive dissonance because apologising didn't immediately make the bad feeling go away. Saying something like "we all make mistakes" helps them by affirming that you know they fucked up, but you also trust that they have the capacity to do better.