Steps to reproduce:
1) Open custom workflow, select some transition.
2) Add Validators "Field has been modified Validator" with field = Original Estimate.
3) Save and select this workflow to some project. Create screen for this transition with "Time tracking" field.
Open task and try to do this transition.
Expected: If user doesn't type Original Estimate, he cannot change status.
Actual result: User can change status regardless Original estimate field. (Field Validator works for custom field)