I've seen it too and only in NR5 but couldn't reproduce it, so I ignored. With your description, I was able to reproduce in NR5. So there, it's definitely happening.
For anyone finding this topic in the future, this has been identified as an upstream issue with monaco itself. We'll keep any eye on when they get it fixed and roll it into a NR maintenance release.
Any chance of rolling back the update of monaco in NR5? After all the editor in NR4 seemed to be working just fine. Were there any specifics errors in monaco that were fixed by upgrading in NR5?