In the user administration, if you edit your own profile and try to create a notification filter, the popup has no backdrop

The problem cannot be reproduced on 10.0 and 9.11.2 because it's hidden by XWIKI-14976. However, these are the steps:

• Go to Administration > User
• Select your own user and click on the pencil
• In the popup, go to the "notifications" tab
• Click on "add filter"

The new popup is displayed, but the old one is not hidden by a backdrop.

I have tried to fix this with usual CSS properties (z-index), but I haven't managed to fix it. There must be a particular reason that make these properties not working as I'm expecting them to do.