Does it trip every time or occasionally? If not every time then run the extension lead tests a sufficient number of times to ensure that the non-fail result isn’t a fluke.
Try the extension lead on the same socket the washing machine is normally plugged into. Does it fail on that?
Also, take the socket off and check there isn’t reverse polarity.
Is this a recent issue or has something been done to the circuit/socket recently to potentially cause a problem?
I would not expect the dishwasher to fail at the same point every time if it’s a broken or faulty socket. It is more likely to happen randomly in the cycle and also with other appliances.
The RCD should pick up an earth fault before the MCB but if the RCD is faulty the MCB will trip if the fault current is big enough.
Rich.