When an instanced UObject property is added to a custom code Component class, changing the value of the property in a Blueprint that includes the custom Component only lasts until the Editor is closed. When the project is opened again in the Editor, the value of the property has reverted back to its default value.
This only happens if a default subobject is created for the instanced object property in the custom code Component class' constructor.
The Instanced Object Rep property has reverted back to "My Instanced Object."
The Instanced Object Rep property is still set to "BP Instanced Object."
Do not create a default subobject for the instanced object property.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-67880 in the post.