I have a job configured with P4 Plugin and using template workspaces. Recently I noticed that the job was not syncing the changes from Perforce. Looking in the console for the jobs I found that they all had a message about the client root being 'null'.
Example:

P4 Task: syncing files at change: 19556
... p4 sync -q /home/jenkins/...@19556 -
p4 sync -q /home/jenkins/...@19556

Path '/home/jenkins/...' is not under client's root 'null'.