It is not a bug, it is a feature. But really it is whatever you are used too.
I am annoyed every time that I use notepad++ (which does not do so).
If I have: Foo.setSome()
And I want: Foo.setOther()
I expect to be able to place the cursor after the "set"..., type an O, open autocomplete and choose setOther, and it should remove the unwanted "Some".
Even worse, if setSome, should change to setHelpSome, if I type the H after set, then N++ offers the full setHelpSome, but choosing this end up with setHelpSomeSome, because the Some was not overwritten.
Of course I understand, if you are used to keep the text, then the removal is equally annoying.