Bosch cordless kettle switch turns of well before water boils?

Robrt Bosh Cordless model: E-Nr. TKW 1201/02 FD:7806
Type: WK 9 - A

The on switch has to be held on for few seconds to get it to stay on when cold and then turns off well before boiling.

The 'switch off' should be triggered by a small Bi-metalic disc/dome but it turns off before this 'pops'

The switching set frame inside looks like it is rivited to the metal base... i.e. not removable to replace.