I’m not getting the same results you are when trying to reproduce this but there’s something from your reproduction steps that confuses me a bit. You mention generating a new C++ class and then attaching it to an object. Does the new C++ class need to be of a component type so that it can be added to an object or can it just be placed in the world? I did the following and didn’t see any issues:
Created a blank C++ project
Used the “Add C++ Class…” button to create a new C++ class based off of Actor named MyActor
Added an instance of MyActor into the level
Added the UPROPERTY declaration along with the variable declaration itself (I just made a int32 called MyInt) and set it to 5 as a default value
Compiled
Checked the details panel of the instance in the level and MyInt was there with the default value and was editable