Flexi Task - outcome is sometimes blank

I have a Nintex approval workflow in SP 2016. I am using a Flexi task within an approval workflow, It has 3 outcomes,

1. Approve

2. Reject

3. Rework

I am storing them in an outcome variable called txtOutcome. It is "All must respond." 

However, when I try to reject or rework, the txtoutcome variable shows a blank or a null value. Not sure why. I do select Reject and rework. There are times it shows value. Acts randomly.

Any idea.

Thanks

Mayank