This is hard to tell as I was only printing 1 layer test prints for calibrating my printer when I reflashed the firmware, and those don't warp. So I don't know if the warping started happening right after the firmware change, to have a good reason to exclude any other possibility.
It's been 2 weeks since I had reflashed my firmware before I started printing anything useful other than 1 layer test prints again to notice it.

But these facts might help:
1) same enclosure
2) same filament
3) same room