Ok, I now understand the issue properly.
The way my multisite is set up is that the MAIN SITE uses a custom theme, and the sub sites use a child theme of that theme. For this reason I don’t think I can simply use the PHP code specifying the fields for the sub sites in the MAIN SITE’s functions.php, or load up the fields through JSON in the MAIN SITE, as I don’t actually want the fields active on the parent theme.
I will have to come up with something else now that I better understand the issue, but I see that this is not actually a bug so it is resolved.
@hube2 thank you very much for your help and gratz on solving 666 o_o
