Updating a user via API resets group membership access level

My scenario:

I have a user who is assigned the coadmin role. He is the admin of 3 groups.

When I call PUT https://api.box.com/2.0/users/***number removed for privacy*** with a body of {"role":"coadmin"}, the API call succeeds and he is reset to just a member of all groups that he was an admin of. He is no longer an admin of any group. Why does this change the user's group membership access level? I would expect this API call to no have any effect.