Substitution rules takes effect on all connectors
This is the expected behavior. Universal Worklist is designed in a way that it will attempt to activate the designed Substitution Rule for all defined connectors, regardless of their type. The reason is that in Universal Worklist substitution rules are defined per user, and not per system. That is why, in case the substituted user or the user acting as a substitute do not exist in a particular system, after creating the substitution rule, the Rule Activation status will state "Problems reported (repairable)", and opening the link of the Rule Activation status will point to the specific error message reported by the Connecor defined for that system.
Refer to a sample error message pointing to missing user mapping below:
Substitution, UWL, "Problems reported (repairable)"
Note: It is recommended that substitution definitions be created and maintained only via "Manage Substitution Rules" in Universal Worklist to ensure consistency in Universal Worklist substitutions.