We're sorry for the troubles! Thanks for reporting.
While we could not reproduce your described problem with your provided repro assets, we found a different issue to occur every second reimport of this directory, so far only on Unity 2021.3 (after deleting the generated assets everty time to start with only the three exported assets): an error is thrown saying that the main texture at the material is null. We will investigate this observed issue and let you know once we have a fix to offer, perhaps it resolves your problem as a follow-up issue as well.
Did you receive any error messages in the console window as well when importing the provided reproduction assets? Could you perhaps provide some reproduction steps how we could reproduce the problem of many instances being added in the Hierarchy window?
