TY.

Dunno what caused it, but we pulled the head off last weekend and saw no any clear damage anywhere. I ordered a GroupA gasket and gonna put it back together next weekend, hoping I'll get some few drives for the motor before the winter comes.
I was feeling a little unmotivated for a headgasket change because I didn't want to pull the head off before I'd be able to put some performance upgrades in. But I don't currently have the parts or the budget for all of them so it'll have to do for now.