After an hour of troubleshooting, I learned the hard way that a workspace will not be deleted if "Delete workspace before build starts" is checked and a "Pattern for files to be deleted" is added and set to "Include" but no actual pattern was specified.

This subtlety can easily cause a bad user experience and I'd like to suggest that if a blank/unconfigured "Pattern for files to be deleted" is specified, the pattern should be ignored.

I've attached a crude screenshot of the configuration cause the unexpected side-effect where my workspace would not get deleted.