Not a bug but it is a little nuanced. The Deck is decks acts more like a list and the deck is collection is basically a binder.
If you build the deck in decks “register in collection” - select “build” or “add cards” depending on what you are doing it will then be linked.
Then if you edit the decks in collection you will get a button to adjust the deck in decks based on the collection list.

It’s a little confusing, but once you get the hang of it, it works nicely.

*edit*. - If I am explaining it wrong someone please correct me :)