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.
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.
When a element is inserted into an array "The value of the inserted element is undefined and should be explicitly set after calling this function." from the documentation at https://docs.unity3d.com/ScriptReference/SerializedProperty.InsertArrayElementAtIndex.html
With that in place we would recommend calling .ClearArray(); on the SerializedProperty when you are expecting it to be clear.