Reading your post, I remember, that I had the same ‘issue’ and found, that reopening worked - so I did not care about that.
Maybe you already found this very old thread, which describes the same behaviour.
Also note that if you create a Blueprint derived from a C++ class, it will appear to be a full Blueprint the first time you open it. You might have to close and reopen it for it to be recognized as Data-only.
So it looks like it is a “works as designed” and nothing did change since then…