Field validation not working well

We are experiencing for the second time that WFG Form lets Requester submit the request without populating field that is required (in our case Hire Date field). This field is set required through Field Validation, not through Form Behavior.
As a result, next participant in the process is prevented to complete their step, since there is required field in the form (Hire Date) that is empty. For next participant, that field (and the whole Requester section) is read only (through Form Behavior) so it cannot be populated additionally by himself, and it is preventing him to complete his step in the process.
Does anybody have any idea why is this happening?
Thank you in advance,

Hi Marina,

Please open a ticket in our Helpdesk site so we can take a look at your process.