Supplier & Orders Dept. Emails Not Sending

Admin side Order Details, Supplier & Orders Dept. Emails Not Sending when press save.
Since updating to 4.3.10, the Supplier Email Notifications are not being sent.

For this particular, website we send supplier notification manually after checking stock etc.

Everything was working fine until the 4.3.10 update. No changes have been made to the email configuration nor server settings.

It seems that emails are sent when the customer places an order, but for Admin, selecting the check-boxes for "Notify customer" and "Notify orders dept" and then pressing [Save] does not sent the emails.

Also noticed that when the customer had changed their profile details, checking the box to save the new details to the order (and then save) did not save the new details.

I have cleared cc and ctpl cache, and tested with mod_security OFF. still not working.

Tested via SMTP sever and PHP mail function.

Have tested the email server and all is good.

Have tested in both the storefront and All stores view mode.

We have a v2.1.x website also running on the same server without issue.

Ive tested this with other suppliers also, so have logged a bug report here: