Hmm, relevant plugins are onesignal and push-notifications and follow.
@outofthebox I think this is related
The user that created the post (user #15356) that generates the notification to the user repeatedly receiving the notification (user #956) is:
- following the user receiving the notification
- followed by a user that does not exist (user #16229)
:
select user_id, name, value from user_custom_fields where user_id=15356;
user_id | name | value
---------+--------------+------------------------
15356 | followers | 16229 --> this user does not exist
15356 | following | 56,0
15356 | following | 956,0 --> the user receiving the notification