The WebFlowConnector does not support "Turn off" substitution, only "Delete"
This is the expected behavior. The WebFlowConnector by its own supports only "Delete" option for substitution. Without having defined a connector that supports the "Turn Off"/"Turn On" option to Deactivate/activate the substitutions, such as the AdHocWorkflowConnecotor or the BPEMUWLConnector, this option will not be provided for the WebFlowConnector.
UWL, WebFlowConnector, "Turn off", "Delete"
As explained above, in case you would like to have the "Turn Off"/ "Turn On" substitution option available for the WebFlowConnector, you need to also have a connector that supports those options.