The question has been popping up on the DL forum as well. When this happened to me in May, DL told me that this happens occasionally, no problem, just have FB reverse the credit and then we can credit it your SkyMiles account. FB told me "we stopped reversing credit a while ago and we don't know why DL told you to ask us to do it because we don't." Brick wall.

Escalating on the DL side eventually resulted in them crediting it to my DL account anyway, but it took a bit of persistence.

Moral: if you want to credit to DL, do NOT pull up your reservation on AF or KL web sites or apps while logged in -- this appears to pollute your PNR with your FB number instead of your DL number.