Hi Senthil,
This will need a deeper look into the workflow design and schema details . The attribute username is not OOTB in nms:recipient so we need to check how the structure is updated and many other things.
It will be best if you can open a support ticket regarding this. Else , please share the workflow logs so that we can guide further.
HINT : You can enable SQL logging to see the exact query being fired on Database and we can have some idea about where the data is getting lost.