I’ve managed to fix this issue. So my website is a multisite & has a Parent / Child Theme setup.
Apparently the only problem i had was that the field group hadn’t been “imported” in the Child Theme. E.G. ACF > Sync Available > Field Group > Import
So even though the fields were actually visible in the admin, it wouldn’t return the array of data to me until it was imported.
I’m not too sure why that’s a requirement but it’s fixed it regardless.
