I would ask the user what steps they took. It may be a fault in the process and not a Workfront glitch. I know with all the Workfront instability lately it's the first place you would look but it may not be the case.
I've had the same kinda thing happen and it turned out it was not Workfront's fault.
Whenever this happens to us, it is 98% of the time because the custom form (or a particular custom form section) was not shared with the requester. So in effect, they didn't see the form (or that section) and just submitted the request without it.
Is this what is happening on your end or were they able to just get around the required fields somehow and filled out the rest of the form ok?
It should also be noted that required fields are a display rule, meaning the rule only applies if the form is displayed on screen. If you have a field from the form on an in-line editable report, they can fill the field in bypassing the other required fields.