>> But I wonder, was this situation discussed in the original bug/issue?
>Doesn't look like it:

I was looking at the wrong PR link. This has more discussion: https://github.com/python/cpython/pull/13305.

nargs is discussed but I'm not sure it was realized that the nargs=None and nargs="?" cases would act in the way seen here rather than acting like append.

Having a default nargs of "+" was suggested but that suggestion was not addressed.

> I suggest that the default nargs for extend should be "*" or "+" and an exception should be raised if nargs is given as "?".

I'm not convinced about that any more. Using append's behaviour is probably more reasonable for nargs=None and nargs="?".