Regression: In the document editor adding a new tag or category won't show until a save

So, while editing for example a News document, trying to add a new tag or category (right column action) seems to result in nothing.
Until the document is saved, then the new (empty) tag/category show up. Which can be multiple if you tried several times before.
The only time it does work immediately is where there is no tag/category yet to begin with. Then adding the first new tag/category shows up immediately, but subsequent entries again not until a save.

This might be a simple bug in the specific tags/categories plugin or maybe more seriously in the CMS core.
Priority then may need to be bumped from Normal to Higher or even Blocker.