How to track the source of null object references in AsyncLoadSceneOperation.ScheduleSceneRead()

I’m stumped with an error in AsyncLoadSceneOperation.ScheduleSceneRead() where one of the entries in objectReferences is null. When we check the contents of the array, the part with the null entry are meshes so we assume that we have a missing mesh in the subscene. We created a script to look for this object in the subscene but came up empty. We’ve checked MeshFilter and LODGroup as possible culprits but couldn’t find one. Any ideas how to track this down?

This is the part of the code which could have null entries:

I have tracked it down to meshes that don’t have assets (procedural meshes). Is this supposed to be supported?