I have a mega options page that has 70 fields and works fine. When I added 2 more fields (72 total) I started getting the “No Custom Field Group found for the options page.” message. Despite there being a group assigned 2 seconds earlier.
Creating a new fields group assigned to the Options page removed the error, but the large group didn’t show — so I tracked it. Still no luck.
Re-saved the group, changed the location options, saved, changed them back, no luck.
Removing 2 items from the large group restored the option page functionality.
I get that there has to be some limit, but 70 seems a bit arbitrary and more like a bug than a performance issue? 😉