Can't remove user from role

I have a user in a database role that I cannot remove from the role.
If I go into the user's properties and deselect the role, click OK,
then go back in, the role is still checked.

I've tried to remove them via sp_droprolemember without success.

I am able to add and drop the user from other roles, though, and I'm
able to add/drop users to the same role.

What's up with that? How do I drop them from the role?
.