Hi John,
Unfortunately that didn’t solve the issue either.
It’s as if ACF is stuck in a state that is styling that meta box to display:none.
I was sent this link where someone else had the same issue.
They give a good explanation to why they thought it happened, but sadly their fix didn’t work either. But I thought I’d include it here for others in case they run into the same problem.
https://wpbeaches.com/wordpress-custom-fields-missing-acf-active/
It’s very strange.
Arron