My understanding of the homeland bonus is that it applies in core provinces as long as you still own the province but does not apply in core provinces if someone else owns it. (The Wiki is vague enough that I am not sure whether this is intended behavior or not.)

You can see in the first screenshot below that the attacking unit, an anti-air, doesn't have the 15% defense normally associated with the homeland bonus (next to the fortress icon in the STATUS column), so it would appear that things are working as described above. [Edit: in case it is not clear, this is the anti-air's core province and I have captured it and am now defending.]



However, even after attack, the defending stack shows 9.4 defense vs unarmored but only did 7.2 damage to the anti-air.



This result is outside the +/- 20% random factor (it's a reduction of ~24%) and could only be achieved through some other way of reducing damage, such as the homeland bonus being applied: 9.4 damage minus 15% homeland bonus would be 8.0, so the 7.2 damage noted would only be a 10% deviation from expectation, which would be within the +/-. Thus, it seems like this AA still received the homeland defense bonus despite not showing it in the STATUS column and despite not being on owned territory.

What might be relevant information is that this AA started its journey towards this province while the province was still considered homeland. I took the province over before the AA arrived.

Is this a visual error, where units are actually supposed to get homeland bonuses even when the province is no longer owned, and the issue is with the defense bonus display in the STATUS column? Or is there another issue at play where a unit that shouldn't be getting homeland defense is still receiving it somehow, possibly because it was sent to that province before ownership changed?

I guess I should have made it clear that this is taking place in a core province of my enemy, which I have captured.

Not yet, I'm trying to understand what kind of bug this is, or if it's not a bug at all and I'm missing something obvious.

I hoped the second round of combat might provide another data point, but the attacking unit died so it's impossible to draw any further conclusions from this interaction. The 7.8 health remaining could have been killed by either the full damage (9.4) or the homeland-reduced damage (8.0).

Funny you say that, because the AI was kind enough to do the same thing a second time and that's what happened:



This time it did the expected damage assuming no homeland bonus. There must be something about updating the status of a unit when a province changes hands, similar to speed, that happens (or doesn't happen) with things like homeland bonuses.