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. We understand this may differ from your expectations or workflow. This field is not serialized. The value is derived from the assigned type, defaulting to Texture2D when null. Leaving the value null will not save the type selection, as it has no functional impact.
We will close this case as 'As Designed.' If you have feedback on how the feature could better meet your needs, please let us know - we value your input and consider it in future improvements.