Is Indigenous Military Outposts bugged?

[QuintusSertorius]

As Epeiros I installed the Phrourion Laon (Indigenous Military Outposts) building in Massalia, and it seems to be making the Hellenistic Polities culture go down (from 29% to 27%, having originally risen from 25% to 29%), and the Eastern Imperial culture go up (from 9% to 11%). Which makes no sense at all.

Is the "Cultural Conversion Bonus" associated with this building going wrong? Like applying in the wrong direction? Shouldn't it be making my culture (Hellenistic Polities) go up?

And where is the Eastern Imperial culture coming from in the first place? Massalia starts out 0% Eastern Imperial, something is going funny here.