When adding markup to the HTML Embed Code Editor in the collection item’s Rich Text editor the markup isn’t saved. When you close the HTML Embed Code Editor and reopen it after saving it the markup isn’t there.
I’ve tested this in the HTML Embed Code Editor in a Rich Text editor that is not inside a collection item and it works fine. The issue is only present when trying to edit it in the collection item (in the designer or editor view).
It also happens in different browsers, new login sessions, and on a client’s editor account.