If the notification helper states the user should get a notification for that event, then typically they should. Are you using Jira Server or Jira Cloud?
If you are using Server, you can follow the steps in Diagnosing Mail and Mail Handlers Issues. Specifically you can enable the debug logging for the outgoing mail. This way if you can reproduce this problem readily, you then will have a log to check in the $JIRAHOME/log/atlassian-outgoing-mail.log that should have a record of the message being sent at that time. This log can be helpful to understand if the message was first generated by Jira, and whether or not Jira got any kind of initial SMTP rejection when talking to the outgoing mail server.
It won't tell us if the client's mail server somehow filtered this message (lots of spam filters can do that). But this is at least the next step in my view to make sure that Jira is actually creating the notification and handing it off to the SMTP mail server.
If you're using Jira Cloud, then it might be better to raise a support request at https://getsupport.atlassian.com with your Cloud URL and try to provide specific dates, times, users, and issue keys involved in these notifications. It might be that our Cloud support would have to change a logging level as well and then reproduce the issue to better understand the possible causes as well.
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