All Live notifications emails are received again after every instance restart

STEPS TO REPRODUCE:

Preconditions: Mail Sending on Administration is configured. Eventually, go to

webapps\xwiki\WEB-INF\xwiki.properties

and set/un-comment

notifications.emails.live.graceTime = 1

to enable the sending of e-mails after 1 minute.

1. Login as Admin
2. Create an user (e.g. U1) with a valid email address in its profile
3. Login with U1
4. Go to Notifications Settings and set Pages toggles to ON
5. Set Email Frequency to 'Live'
6. Login as Admin
7. Create/update/comment a page
8. Observe the emails received by U1
9. Restart the XWiki instance

EXPECTED RESULTS

No emails are received after the instance is restarted, since all notifications emails for events were sent already.

ACTUAL RESULTS

After restart, the user receives again emails with all events that took place all the way since the Email Frequency preference was set to 'Live' (even events related to Administration settings made by Admin: Mail_Config_event.png).

Every time the instance is restarted, all emails are received again, which can cause important issues if there are a lot of events to be sent (and to multiple users).

In console, there is the following WARN when the mails are sent:

2020-11-04 13:05:45,354 [Mail Prepare Thread] WARN  o.h.e.j.s.SqlExceptionHelper   - SQL Warning Code: -1100, SQLState: 02000
2020-11-04 13:05:45,354 [Mail Prepare Thread] WARN  o.h.e.j.s.SqlExceptionHelper   - no data

This issue could not be reproduced on XWiki 12.5.1.