If you edit an existing artist credit and lookup an artist, it will change the name the artist is credited as. I don't think it should do that if there's already something in that field.

For example, let's say I'm crediting something to "alan". I enter "alan" in the lookup, accidentally pick "Alan (Mexican artist)" but don't realise. It's a Chinese release, so I enter "阿蘭" in the credited-as bit, and then notice I selected the wrong artist. When I look up the artist again and select the right one, the credited-as field changes to "alan", losing what I originally entered.

Instead it would probably be better to have a "copy" link/button to the right when there's already something in that field (like on the edit artist page for sortnames)