Steps to reproduce:

1. Create a job with P4 Plugin
2. Add two parameters to the job
   1. changelist (leave default blank)
   2. depot_path //depot/some/path
3. Set Manual View Mappings:
   1. ${depot_path}/... //${P4_CLIENT}/...
4. Set pin build at perforce label:
   1. ${changelist}
5. Check "Poll SCM":
   1. EG: H/2 * * * *

This build will run perfectly when running via "Build Now"  But if you watch the Polling log you will see it continually fails with:

"Mapping 'depot_path/...' is not under '//depot' 

If you change the View Mapping to a hard-coded depot path you will still see:

P4 Task: failed: com.perforce.p4java.exception.RequestException: Invalid changelist/client/label/date '@changelist'.