Custom AssetBundle dice assumes D6#2150

I’m trying to make a custom D8 with particle effects. I built the die in Unity as an AssetBundle using features that are not available in the built-in Custom Die Maker:

• Importing an AssetBundle will only allow rolling the object when selecting the “Dice” import type.
• Selecting the “Dice” import asset type assumes a D6 and applies a Cube collider to my bundle, despite the fact that the AssetBundle has an appropraite collider component. This results in my die standing on end or on edge instead of landing on a face.