When using the route Resources -> Add Digital Representation, you can search for and add a representation to any existing collection. However, if you have a bib record with multiple collections, and you use the "..." -> "Add Representation" menu option on that record, the representation defaults to one of those collections and you're unable to change it; this forces you to use the first route instead if you want to add that representation to a different collection.

Using the "..." -> "Add Representation" route should allow you to select which collection you want to add it to if there's more than one collection on that record.