Incorrect read status and grouping of Word-Based notification events in the list

Steps to reproduce

1. Login as Admin
2. Create an user (U1) 
3. Login with the user and on his profile, on "Word-Based Notification" tab, insert a query (e.g. "QueryA")
4. Login as Admin
5. Edit <server>/xwiki/bin/view/Sandbox/TestPage3 and insert the query (e.g. "QueryA"), then 'Save & View' the page
6. Add a comment to the page (to generate a different event) <server>/xwiki/bin/view/Sandbox/TestPage3
7. Login as the user and observe the Notifications list
8. Mark the Word-Based Notification as read (which is displayed under the comment in the list)
9. Login as Admin
10. Edit <server>/xwiki/bin/view/Sandbox/TestPage3, insert again the query (e.g. "QueryA") and 'Save & View' the page
11. Login as the user and observe the Notifications list
12. Click on a "Mark event as read" button from the Word-Based Notification events

Expected results

The Word-Based notification marked as read at Step 8 (first query occurrence) has still status read and is displayed under the comment event in the list (because is older than the comment), being separated from the second query occurrence event (which is first on the top of the notifications list).

Actual results

The notification marked as read at Step 8 (first query occurrence) is displayed in the list grouped with the second query occurrence (from Step 10) and displayed as it appears to be a single entry (no separation dotted line between the two events).

Also, both Word-Based events notifications are displayed at the top of the list now (regardless that the first occurrence was before adding the comment - at Step 5).

When clicking on a "Mark event as read" button from the two buttons available, the whole entry (composed from both events) becomes read, but the other "Mark event as read" button is still displayed.

It is to be mentioned that on XWiki 15.5.3, when the Word-Based notifications events are grouped, only the first "Mark event as read" button is displayed correctly, the rest are displayed out of place as a small button that does nothing when clicked (see screenshot).