I don't believe multiple maps would be a problem if you only have one enabled, and personally I've never encountered a problem with multiple maps showing at once, other than it slowing everything down a bit.
Your screenshots show that the 'detail' is actually still there - the road names are still showing, and the name is just an attribute of the road, so it's there somewhere, just not being rendered. That could possibly be caused by having two maps enabled and the one on top showing less detail and partially blanking the one underneath. But it's not a theory that convinces me.