We are seeing the same issue.
We have around 10 WordPress sites hosted on two different servers/vendors, with all plugins and WP up to date. The Mail on Update settings have been checked, and everything is configured correctly.
The WP sites are sending all other mail for user registrations, and so on, but Mail on Update is no longer sending any email. Checking the logs shows that there is no attempt to send mail at all; no messages are being blocked, filtered, etc. No messages are being generated at all by Mail on Update.
This plugin has been fantastic until now, so we hope there is a fix.
If anyone has a fix, we would be very grateful to learn what it is.
