I’m having the same problem. I had no choice but to create a boolean global variable and setup the validation manually using workflows and conditional logic in the submit button.
By comparing a field that works with the one that does not work I might have found the origin of that issue. @avidflow and @jose could you please give feedback if that is the issue for you too?
When I use the binding button (clicking the plug symbol) to add the custom validation formula, the validation result is faulty.