I know a lot of autistics struggle with facial expressions, and I thought I did too, but I discovered recently that it's more complex than I thought. I've noticed that if I'm not presented with a social situation, for example if I'm watching a documentary or video of some kind, I can read the facial expressions better (I think) though I have no evidence to back it up given that I'm not directly communicating with the people I'm watching.
However, if I'm in a social situation and expected to talk and socialise which I really struggle to do, the attention I have to put into socialising and talking takes up all of my resources and peoples facial expressions stop being obvious to me. For example, my partners parents came to where we live this weekend, and I noticed that because I was focusing on trying to socialise or to come across normal, my brain wasn't able to calculate what their faces were saying whilst they were talking, and I stopped being able to respond accordingly to what their faces were saying so I probably made wrong comments or gestures.