Ok, after spending far too long on this I have discovered the cause of the empty UMC, at least in my case.
In the configuration registry the setting ‘apache2/force_https’ must be unset.
I had this set to ‘yes’ on the two DC’s with the issue, however, on the servers without the issue this setting had no value.
After removing the value and running ‘systemctl restart apache2.service’, the UMC is instantly populated correctly.