I have just updated Scrivener to the latest version, then continued with some work I was on. On selecting some text, I hit the “bold” formatting button and the text disappeared.
“Undo” did not bring it back.
However, closing Scrivener & re-opening it showed the text in place and in bold.
On playing around with this bug, I found it seemed to happen only with the title text in the editing window. Text in a paragraph was successfully changed to bold without this phenomenon. (By “title text” I mean the first line in the text editing window that changes the “Untitled” in the Binder to the wording of the first line.)