Restricting open enrollment based on union and non union programs

Summary:

We have two sets of employees union and non-uion and have respective
programs for it. When we run the 'Evaluate Scheduled Event
Participation' process, client wants to restrict only for union and we
selected the program name and plan type name to help restrict for only
union but it does not restrict and runs the open enrollment for the
entire population.

Is there a solution or workaround to deal with this scenario.

Is there a solution apart from the benefit group? like we selected the plan type, program name so it ideally it should have run only for people who are eligible for those plans and plan type.