DFF value not capture in the fast formulas

We have a requirement to capture the value in the DFF deployed on the assignment page into the global absence entry validation fast formula. But values are not captured to the relevant DBI.

Flex Filed Code = PER_ASG_DF

DFF table value is = ****_ATTRIBUTE10

so I set the default value in the FF as PER_ASG_ATTRIBUTE10

DEFAULT FOR PER_ASG_ATTRIBUTE10 IS ' '

but for the above DBI it returns a null value

Note: Value for this DFF appears in the front end and as well as in the assignment table against to employee's active assignment record.