This really stumped me too – as my scripting looked to be pretty much ok. But I’ve fixed it – it must have been some weird little bug in my system.
I updated my version of WP to the latest (just in case 2.83 > 2.84) and deleted all my previous entries for custom fields. I re-inputted them and it’s working! I have no idea what the exact cause was – but it was worth the try :-)
Thanks for that!
Have a great weekend!