Script for calling timestamp value only works with readonly enabled.

It seens as it can be only achieved when readonly=true is true for field B. When readonly=false for field B, value set for field A is not shown on field B. Customer advised they would like to know if it can be achieved without having to set field B to readonly.
One answer

Comments

Thank you Ralph for prompted answer. I have not retrieved full detail as why customer would like to leave the field as non readonly.

I have used calculated values without setting the field to read only in the past, so I think the design should allow this. I have however no information if that is a design intent or if there are exceptions to the rule for certain types. I would check if the dependencies to the attributes you calculate from are set correctly and try it again.

I have double checked and everything seems to work fine. If dependencies to the attribute I am calculate from are not set correctly, should it not work disregard of "due date" attribute being read only or not?

You should only set dependencies to attributes that should trigger the date to be recalculated if they are changed.
I am not aware of limitations, but you could create a work item to get more information.