Subclass - Proficiency or Expertise Issue?

I'm trying to create a rogue subclass which grants proficiency in Deception, or gives double proficiency (expertise) if you are already proficient. I've used the Scout as a model for how to do that. Under "Add Modifier" I've assigned two - one to give proficiency, and a second for expertise.

The character I'm adding this to is already proficient from his background. When I choose the subclass though, it de-selects Deception from the background and tells me to choose another proficiency there.

Have I found a bug or am I doing something wrong?

Link for mods: https://www.dndbeyond.com/characters/subclasses/3036-cosis-trickster

No idea then? Anyone? :-(

I too am trying to create a Race that can either have 2 Skills or Expertise in one.
Anyone with any advice?