Views:
Behavior: 
A Task decision was created where in the Assign Data section, a User of a Group global variable is mapped to the system variable Last Decision User.
When a user confirms the decision, the variable is blank and not written with this user's name.

Solution:
By product design, the last decision user cannot by acquired as the task is being confirmed. When capturing users from a system variable such as "Last Decision User," it is best practice to capture this data from an Assign Data activity right after the Task. 

KBA is applicable for both Cloud and On-premise Organizations 
This article is valid for DocuWare versions: 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 7.10, 7.11, Global Variable, Tasks, System Variable