Old Prussian not an accepted culture in Kurland

[sr902]

I've played a few games as Kurland, but this was my first one since the last expansion/patch.
In the past, when you form Kurland, Old Prussian becomes the primary culture, but that didn't happen this time, my primary culture is still Prussian. And since there are only 2 very poor provinces with Old Prussian, it isn't even accepted.

And to make things worse, I can't even spread my culture to those provinces, because it says "Cannot change culture while Kurland exists". Is this a bug or some kind of weird new change?