Bug: Certain player slots are being treated like Observers in-game
You are unabled to selectively send a private chat to the player sitting on slot 22 (white) inside a custom game, I cannot confirm if it happens on slots any other of the slots above 12 (it is likely only above 12 because patch 1.29 introduced new player slots and that may have been a bug since their introduction) as well, the one custom map that I play that has that slot enabled has that issue, but I remember seeing it before in another map.
Another interaction that you cannot do with “white” (22) player is send resources through the trading tab such as gold or lumber, again I cannot confirm it if happens to other player slots that are above 12 (new player slots added in 1.29) but there definitely seems to be a bug with it.
The image of the player properties in that custom map:
i.imgur.com/wMnZ07M.png
Reproduction (In Custom Games, not LAN):
Join a map that contains slots red (1) and player 22 (white).
Press F12 and attempt as player red to send a private message to player 22 white (he will not be able to read them).
Press allies and attempt to trade resources with him if he’s your ally.