I found out that Set field by name does not work in Find Mode on calculated fields (while Set field does).
It returns error:  missing field….
That resulted by testing this code:
Set variable [$selection; Value: <my customized function>]
Set variable [$fieldName; Value: Get(ActiveFieldName)]
Enter Find mode 
Set field by name [GetFieldName(Evaluate($fieldName)); $selection ]
Perform Find 
The above code works fine on standard text fields. Not on calculated text fields.
In fact I found a different and simpler solution for the example above using Set field that works in both cases. So this is not the issue.
The question is:
why Set field by name and Set field have different behavior in Find Mode over calculated fields ? does it make any sense or is it just a bug ?