I had the state atheism trait and got vassalized after losing a war, which forced me to adopt the liege's religion (Buddhism). Later when I got my freedom, I noticed that my state religion is still Buddhism, and due to me having the atheism trait all the options related to religion were locked. As the trait is also irreversible, I am effectively locked in to being a Buddhist.

I would expect my state religion to return to atheism after regaining my freedom.