Same here. The problem is with 5.1.9.1. Rolled back to 5.1.8 and it works again.
In my case here’s what happened:
– I could use “Screen Options -> Show on Screen” to reactivate an ACF meta box.
– That would work for the specific page
– If I navigated to another page and reactivated its meta boxes, meta boxes for all other pages would disappear again
