Scalar types works differently other than UID type. If you want this behavior, please fill up an issue.
The problem here is that in Scalar types, Dgraph overwrites, but leaves no artifacts/duplicates/things behind. For example, if you overwrite a relationship. Dgraph would only deal with the relationship, leaving the entity removed there intact unnecessarily.
Imagine you doing it thousands of times, you will have hundreds of thousands of nodes dangling around. With no use.
So, dealing with relations and entities in Dgraph is a thing that the user must deal with. It can be tricky to create a determined behave and not break something. Thus making the user complaining about it.
However, please fill up an issue for the case.