I was trying to use the Perforce Plugins parameter substitution in the label field and couldn't get it to work until I changed my parameter name to all lowercase (string parameter of a parameterized free style build). I would have expected it to work with uppercase letters too. If there is no time to fix this, at least the help should be updated to point out the limitation.
Another thing I noticed is rather a feature request than a bug I think. Leaving the parameter empty to force a build from top of the tree doesn't work because it still tries to specify a parameter. You can see the attempt to sync to '//depot/...@' which fails. Adding some logic to sync to //depot/... instead when the parameter is empty would be helpful.