Board Thread:Support Requests - Getting Technical/@comment-47009466-20200928203216/@comment-45117243-20200928210920

Indeed, I see the problem, and has encoutered it before.

As a temporary solution - when I click the EDIT SOURCE on the dropdown of the EDIT button, it seems to show up well and to remain even when switching to the visual mode.

At the same time, I would recommend you to report this as a bug through the official form Special:Contact, and also to put it on the unofficial document for tracking UCP problems User:Noreplyz/UCP.