I also ran a test with the TEC (still in the test build) to make sure the issue wasn't exclusive to Advent, and found a deeper issue that feels like another bug. Culture push is being "reflected" back to the source planets from neighboring planets. (My expectation was that culture spread is calculated only from the sources, and not indiscriminately from all planets under cultural influence, as that could get messy and blow up real fast. If this is actually the intention then I guess you can disregard the rest of this, although it does still result in some weird unexpected values.)
To eliminate other variables, I set things up such that my home world would be the ONLY place with any source of culture push (no broadcast centers or trade ports anywhere else, and I skipped the tech for culture from trade ports anyway). With two broadcast centers in the early game and no other tech modifiers, I expected 2.0 culture push from the homeworld. Which was true, until my neighboring planets reached 100% culture, at which point it jumped to 2.2. [ (2.0 / 3) / 3 = 0.2222, and 2.0 + 0.222 = ~2.2 ]
Later on, after researching the prerequisites for critical mass (but before critical mass) I was expecting 3.75 culture push (rounded to 3.8) due to the 2 broadcast centers +1 from commerce level and +25% overall, but instead saw 4.2 at the home world. After inspecting the other planets I saw 1.3 in the adjacent planets, and 0.4 two steps away. Those two are what I would expect from a 3.75 source at -200% spread decay, not a 4.2 source (3.75 / 3 = 1.25, 1.25 / 3 = 0.4166....... and of course 3.75 + 0.41666 = 4.1666, closing the loop on the discrepancy).
Further, upon researching critical mass, the culture spread rate at the home world went up to 4.7, which is certainly an unexpected effect when that's the only culture source. [ (3.75 / 2) / 2 = 0.9375, and 3.75 + 0.9375 = 4.6875 ]