Thank you for reporting a bug to Unity.
After reviewing the behavior, we've confirmed it aligns with the current design and intended use of the feature. The core problem here is that baking with emissive objects will always require significantly more samples for a clean bake. There are 2 possible ways to go about it:
a) Increase indirect sample counts significantly and enable APV dilation. For the scene attached this could mean up to 8k Indirect samples. Bakes will take significantly longer.
b) Make sure emissive objects are not static, effectively not participating in the bake. Place baked point lights in their locations to imitate the lighting. Faster bake times with fewer samples, but will require some re-authoring.