This is still an issue. Since ‘staff’ is an existing group name, setting the min username length to anything other than 3, 4 or 5 will break granting and revoking admin if you do not rename the staff group first.
Worse, the error message is just “The change you wanted was rejected. Maybe you tried to change something you didn’t have access to.” and the logs show “Name must be at least 6 characters”. Took me while to find out that this was about the group name (no actually, I never figured it out, I found this topic 
).