I’ve managed to solve this issue.
It appears to be some kind of conflict with Post Types Order. Disabling PTO fixed the issue.
https://wordpress.org/plugins/post-types-order/
See https://wordpress.org/support/topic/problem-with-acf for reference.
I don’t know which program is the reason for it but I’m using ACF Version 5.1.6 and PTO 1.7.4