LPG Hot Water tank pop-off valve erupted - is this normal?

Thanks everyone for such quick replies!
It is a Suburban and on the control's top it says "sit 610-AC3". It has a black knob that says "off, Pilot, On" and then in the center it has a selector that lets me pick one of five temperature settings, "High, Medium, Low" with notches to either side of medium. It overflowed whilst on Medium.

We replaced the thermocoupler originally because it wouldn't light and, once we replaced it, it lit no problem.