In

_ActionsContainer._add_container_actions there is this note:

    # add container's mutually exclusive groups
    # NOTE: if add_mutually_exclusive_group ever gains title= and
    # description= then this code will need to be expanded as above

In other bug/issues we've noted that the way to give an exclusive group a title and description is to nest it in an argument_group.

So it looks like no one has worked through the details - in this parent situation - of adding a nested mutually exclusive group.

In my preliminary tests, it looks like the 'group_map' is being messed up by the block of code that adds the mutually_exclusive_groups.  I don't know how easy it is to fix that.

The immediate user solution is not use parents and nested mutually exclusive groups together.