I was at a bank once when this uncle came to withdraw all the monies from few of his joint account with wife.
After he left,the teller said he suspected the wife passed away but he just pretend he have no knowledge of it.
But if he wanted to follow SOP, and after enquiry, if it is known that one of the joint account holders is deceased, they are not supposed to released the money until the proper documentations are obtained .

If this is correct legal position, I am wondering how many tellers will follow SOP and how many just act "don't know"?

Not sure about front counter banking operations. But teller should or would not ask such question. Instead they can see from the system if there’s a court order to stop the withdrawal.

Legally if I understand correctly, if A and B opens up a joint account and A has passed on then the money is split 50 / 50 between A’s estate and B. B is allowed to withdraw all amount BUT if required by law B would need to return the excess of the 50%