OK, so a very helpful user PM'd me on here and pointed me in the right direction.
This file:
LibreELEC.tv/proftpd.start at master · LibreELEC/LibreELEC.tv · GitHub
Is what is overwriting the permissions. You can go in here and comment out the umask part and that will stop it overwriting the umask.
This allowed me to set the Umask to whatever I wanted, however I believe when the software gets updated, this file will be rewritten and I'll lose this fix.
My issue actually turned out to be an issue with LFTP which I resolved, so I won't be going any further with this. I just wanted to highlight it as it's obviously going to be an issue for others who want to use advanced proftpd settings.