It’s a bug. Please file an issue. When you check the “show non-sematic” check box, you should see allItems no matter what they do or do not have defined on them.
I opened an issue some time ago and Florian said, he is not able to reproduce the problem and it seems like nobody else has this problem. Is there away to reset my UI so it behaves like a fresh install?
I imagine you can start with a brand new OH instance and then bring your Items over one by one until the problem occurs. Then you can examine the Item that causes the problem and perhaps it can become reproducable by others with that information.
It took me too much time to start from scratch again. I guess I will remove all metadata, when I want to create a rule and add them again afterwards. Very painful but better than starting allover again.