Live email notifications are received in a single email at once instead of a separate email for each event

Preconditions: Mail Sending for the instance should be configured from Administration.

Steps to reproduce

1. Edit xwiki.properties and set notifications.emails.live.graceTime = 1 to enable the sending of e-mails after 1 minute
2. Login as an user (e.g. U1) which has an email set in his profile
3. Click on Notifications Bell and set all Watch toggles to ON
4. Go to Notifications Settings
5. Set "Email Frequency" to Live
6. Login as Admin or other user
7. Perform some actions (e.g. create a page, edit it, delete it, comment in another page, mention the user U1)
8. Check the user's U1 received emails

Expected results

The notification events are received in separate emails, sent as soon as they are finished (an email for each event is sent after the set time passed, 1 min in this case).

Actual results

The notification events are received in a single email at once after the time set passes.

The issue could not be reproduced in XWiki 14.10.13 or XWiki 15.4.