Nick commented on 36844, comment 9: "I found that when switching from a branch with a default register to one without, the default register for the last branch was remaining selected (though disabled)"h When switching from library A to library B the cash register should switch to register B or none.
Related, if the default register does not appear in the dropdown at all. This breaks the user menu switch as well, since it will direct you to the page to switch branches since you did not select a register.