Some apps request notification permissions and register a service worker that is listening and showing these.
Because apps are backed by plain tabs in the background, if you grant notification permissions to one, depending on what the app does, it can continue to show notifications.
This is the same as in other browsers, where a tab can request notification permission and then continue showing notifications after you've navigated away from the tab.
You can turn these off on a site-by-site basis by doing the following...
1. Open wavebox://settings in a new tab
2. Click on the "Privacy and Security" tab in the sidebar
3. Click on "Site settings"
4. Under the "Permissions" section, locate "Notifications."
5. Under the "Allowed to send notifications" section, locate the app you want to remove and use the three-dot menu to remove it.