Some in-app Notifications and emails not delivered for POs intermittently

Summary: Some PO approval notifications and in-app notifications are not being delivered for POs

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Users are complaining some in-app notifications and emails are not being delivered to members of the approval group. This seems to be an intermittent issue and some members of the group are receiving the in-app notifications and emails, so the POs are getting approved, but with some delay. There is no error being thrown and if I look at the POs, the correct recipients have seemingly been emailed etc, its just the users are saying they are missing notifications that their approval group member colleagues have received. This seems to be since 25A. We have been live 5 yrs and