Expressions entered in the sheet description field using the expression editor disappears when clicking Apply in the editor, because the sheet title and description editor closes at the same time as the expression editor closes, with no possibility to save the sheet title/description edits. It doesn't matter if the sheet is in edit mode or not.
Qlik Sense Enterprise, tested in November 2022 as well as February 2023 Patch 2. I don't have access to SaaS so I haven't tested there.