Good to see the new Multifield. But, the RTE issue isn't limited to Multi-field though. If you have a tab with multiple RTEs (Title, Description, Subtitle, etc), you run into issues. The main issue I've come across is that when you select text in the textarea, and then try to perform an action from ...

CoralUI3 RTE is broken in 6.3 GA in some instances such as multiple RTEs in the same dialog or multi field. They issues that I was having all seem to be resolved with 6.3.1.2. So if you're having RTE issues in 6.3, install SP1 and CFP2

I ran into this issue today using AEM 6.3 with Oak 1.6.3. Resolved it by running offline compaction. Not sure why this happened. I had added some package dependencies and then removed them before the issue started.