The regions in which I was experiencing a decrease in culture were mostly core regions, that I owned from the start or soon thereafter and were surrounded by other Bulgarian regions for the majority of the campaign. I am yet to see the AI attempt to use agents. It is not spamming assasins like other mods and my spies never had issues infiltrating, apart from the odd general with anti spy traits. There was an earlier post which ran into the same issue, and it may have to do with the fractional bonus churches of level 2 and 3 give to conversion:
https://www.twcenter.net/forums/show...1#post15868609