Reformation

[Trin Tragula]

All the scenarios presented thus far are reasonable, but it doesn't change the fact that the Protestant reformation was a direct reaction against the Catholic Church, not the Orthodox Church. The game shouldn't treat these provinces as eligible to receive the effects of the reformation..

The province probably isn't 100% orthodox though. Perhaps a whole bunch of protestants fleed from a catholic country and moved to your province due to you not distinguishing between catholics and protestants? They could along with a minority of catholics that converts be enough to switch province majority.
Such movements did occur on a large scale (for instance at one time Berlin is said to have been 2/3 french due to the large influx of french protestants).