The results for GT Subset are different for DB2 table which includes generated/computed columns which datatype is timestamp as:
It seems that when using "Workflow,Save Workflow Subset" is a defect.
Release : 4.8
Component : CA Test Data Manager
This "Workflow,Save Workflow Subset" part does not transfer the data for the columns which can be computed (like generated/computed column), because values of these columns will be computed again in the target table.
Then this works as designed.