Hi Vivek,
This is a known defect and is currently being worked upon by engineering
CA Ref : DE280382
Current Behaviour with FormPost variables:
FORM Post variables is NOT resolved , when it is not used in expression of a domain policy.
When Expression is created with FORM Post variable it is being resolved correctly.
So for FORM Post variables, with expression in Domain Policy flow is working fine but its not working without expression in Domain Policy.
So, at the moment you can try to use it in the Policy expression as an workaround until SE fixes the issue.
Regards,
Ujwol