I agree with lorelec. A brief transient to or below 11 might be enough to trigger the alarm but not able to be displayed by the volt meters. The manual says the alarm stays on for 5 minutes though it isn't clear to me if it is referring to the low voltage alarm or the low voltage shutdown which also triggers the alarm (maybe both). Measure the voltage directly at the inverter terminals while someone turns on the coffee pot. If you see 11.5 or 11.6 in agreement with the other meters then it is either a quick transient or there is a problem with the inverter.
I have the same inverter and have run the MW at over 130 amps. But with four batteries I haven't dropped to that low of voltage 'yet'.