Generate Data - not generating random strings but saves a default value

I have a Reference field, where I am entering a random string (min 1 to max 20 chars) and I need this to be unique with a combination of 2 other fields.

I've created this variable using Data Generator > String type, using a combinaion of numbers, letters and space. On the 1st test run, it generates a random string. But after this, it saves this value as the default. So on all my subsequent test runs, instead of generating a new random string, it is using this default value and so it is not a unique reference. I used the 'One value' variable type.

Can anyone suggest what I might be doing incorrectly?

Thanks!