The background of unread events displayed with "Load older notifications" is not grey anymore, but white

Steps to reproduce

1. Create an user that is Watching the wiki (has Watch Notifications toggles ON)
2. Login as Admin
3. Create, edit some pages to generate more than 10 events
4. Login as U1
5. Click on Notifications Bell to open the Notifications List
6. Observe the background color of the 10 events displayed by default
7. Click on "Load older notifications" button
8. Observe the background color of the older events loaded

Expected results

Both the events displayed by default when the list is opened and loaded from older ones have the background grey, since they are unread.

Actual results

The background color of the older events loaded is white (same as read events), even if the events are unread.

The same issue reproduces on Dashboard's Activity Stream.

The issue reproduces also on XWiki 15.9, but could not be reproduced on XWiki 15.5.4 or 14.10.20.