We thought this was the case, and did actually test the family. It didn’t break using Revit’s UI.
However, we suspected that something funky was happening in the family, and that it wasn’t an issue with Dynamo.
We made one of the three instance parameters into a type parameter, and tried again. This time it worked as intended.
I’m not entirely certain what the issue is, but what I am sure about at this point is that we must be very careful about how we model families, otherwise our Dynamo graphs will probably run into this issue again. Have you run into this issue in the past?