Depending on the age of the boiler it could have the rubber primary hoses (later on Viessmann upgraded these to copper due to problems yet they say they haven’t had any 😂)
There not smooth on the insides and go hard so collect smoo let’s say and dumps it into the plate as it’s the nearest thing
It’s dumping the extra heat in the boiler to prolong the life of the heat exchanger
My honest opinion 3 ways don’t need an auto bypass (does this get warm when it does the 3 minutes cycle ?
That’s easy then
Perm live to live on boiler
Neutral to neutral on boiler
Earth to earth on boiler
Pump to pump live on boiler
Oranges from the zone valves to lr on boiler
If you want a read see link below but basically it’s a reaction between the copper and the oil they put in natural gas, and it flakes off once built up
https://www.heatgeek.com/black-dust-sulphidation-in-gas-lines-and-appliances/
I think it’s a main pressure issue as the pilot was lit (eg nothing controls that other than flame or thermocouple etc) so I suspect hot water demand full flame and it’s struggling for gas flow so pilot gets small and boiler locks out
So best to get your gas guy to inspect the gauze in the gas...