In the movie Joker (2019), we see Penny claiming Arthur to be Thomas' son. We have Thomas refuting these claims as well.
We later learn that Penny was in an abusive relation and that she suffered mentally and physically. It seems like she was deluded about the father of Arthur. It could really have all been in her head.
It could also simply be Thomas covering his tracks and planting those records against Penny, which is really not a big task for a man of Thomas' stature.
Given all this, who was telling the truth about this whole scenario (Arthur's father)? I did watch the movie till the end and didn't miss any scenes but I was clueless who was telling the truth and I don't think this was addressed.