Why is this mate?

Hello,

I have a quick question about this game:
https://www.chess.com/4-player-chess?g=4352658-145

Why is this variation immediate checkmate of red by blue?

37.b Qj4+#

Because after blue, it's still yellow and green's turns, and after

37.y Qh12
37.g Qxj4

red could have moved the king to i1 (37.r Ki1?).

Interestingly,

37.b Qi4

does not result in stalemate of red by blue (which is probably because

37.y Qxi4
37.g Qh2+#

can still make it checkmate, but still - I do wonder when stalemate/checkmate is determined).

A player cannot assume in FFA that another player will assist in preventing the checkmate. As it would have stood after [37b. Qj4], the red king would have had no sure way to get out of check and thus be mated. This is in line with the rules. Checkmate and stalemate are determined immediately.

Only in team 4-player chess does it wait until your next turn to decide if you are mated.

Thank you, so this seems to be working as intended. Where can I read about this in the rules?

Thinking further about the statement "Checkmate and stalemate are determined immediately", I cannot fully agree - see the example in my second post (37.b Qi4). This should be stalemate if determined immediately, but it is not.