Same here, we confirm it happens in Chrome. Tested on several Mac & Windows machines with updated browsers.
We noticed it last week. Not sure what exact conditions triggers the hung. My first thought was it had something to do with pasting in redactor, because it happened when pasting and deleting the pasted image/text.
I'm using Chrome in Big Sur (11.6.5) Chrome (101.0.4951.54). Our production environment uses osTicket v1.12.6. However, I checked in a fresh 1.16.2 installation and it happens too.
As someone previously mentioned, the "Post Internal Note" form works fine for us.