A new problem is progress. 
Custom UI and UI Kit render differently. The @forge/api is not available in the Custom UI context but it is for UI Kit. (Same problem as this other thread.) Glad you could switch to UI Kit.
Meanwhile, for our new problem, that’s definitely a scope problem and is beyond my ken. I’ve pinged the PM on the issue, but it might be worth opening up a developer support ticket to see if you can get a more direct answer.