Unable to make changes to existing notifications in vm-notify module

Once a Notification has been configured on a mailbox, I am unable to edit or delete it.  When I click to edit it, the 'Edit Voicemail Notification' page opens and I can edit the fields.  When I click submit and then go back in to verify the changes have applied, my changes are not there.

Similarly if I click delete on either the Notifications list page, or the Edit page, I get a prompt asking whether I want to delete, but when I click Ok, the page refreshes without having deleted it.

Adding a new notification on a new mailbox works fine, all of the values I enter are saved, and the process of getting a call when a new voicemail is received works as expected, I just cannot edit or delete an existing one.

I have attached screenshots of my steps

I have discovered as a work around, that I can update the numbers to be notified for a mailbox, by using the 'Voicemail notifications' section in 'Extensions' module under the 'Voicemail' tab.  Changes made here do appear back in the 'Voicemail notifications' module - I just can't edit them there.

I don't see any errors in the freepbx log when I try to save - I only see a warning ' [WARNING] (enc/functions.inc.php:357) - Depreciated Function vmnotify_display_recipients detected in /var/www/html/admin/modules/vmnotify/views/form.php on line 147'