Zombies Catch Fire Under Trap Doors

So I have a miniature zombie farm design I was testing, where zombies would go after a villager in the middle, and the path finding would see an open trap door as a full block. However, when zombies fell in, they started catching fire despite the rain. I theorize this is because trap doors allow light through, but stop rain - even though they are open. I am uncertain to as if this is a bug, or a scenario the game wasn't understanding how to deal with. Either way, it is an interesting interaction I thought should be mentioned.