It sounds exactly at the level of my ex(he was diagnosed BPD) ... he would too turn it over to me whenever he lost it over something that made no sense to me. Or was even completely misheard/ understood by him.
It happened once (with a lot of patience and me repeating what I said) he finally understood what I was actually saying and he admitted he made up a whole narrative that wasn't true in his head. It was shocking to me because it never happened before, nor again after that one time

But then that would tip over as well into him continuing saying : ' the fact I make things like that up and falsely accuse you makes me a sick, SICK person.'
Which resulted again in me trying to convince him he wasn't and it is not that bad.
It is agony because no matter which way the argument goes it ends up in him feeling shame and rejection. I felt powerless and at the same time invisible.
I guess, as stated above ..keeping calm and validating his feelings are the only things you can do. And hopefully get to a point where he sees more clear what he is doing.