Steps to reproduce: 1. Type a few characters into an empty document in Writer. 2. Format -> Character -> Hidden. 3. Try to toggle the related options, e.g. Toggle Formatting Marks (icon), Options -> LibreOffice Writer -> View -> Display Fields. Expected result: Hidden text is shown with a dotted line below it. Actual result: Nothing is shown. Extra note: This is a regression of LibreOffice 7. I am sure that hidden text is okay before upgrading from LibreOffice 6. Please do a bisect to find the exact commit involved.
Update: The correct options should be LibreOffice Writer -> Formatting Aids -> Hidden characters. Closing this myself.