Add-ons with a Duration are able to be assigned to specific staff members at the business. The add-on will then only be able to be scheduled with the staff it has been assigned to.
But, Add-ons without a duration are assigned to Services, and Services are assigned to Staff Members. So any Staff Member that is assigned to a Service, will also be assigned to any Add-ons without a duration that are assigned to that Service.
As a workaround you could create two different Services. One with the Add-on(s) and one without. Then assign those Services to your Staff Members based on whether or not they can provide the assigned Add-ons.
Adding Services to the Service Menu
For example, Staff A can provide all Add-ons but Staff B cannot. You can create a Service titled 'Massage with Staff A', and a Service titled 'Massage with Staff B'. The Add-ons would be assigned to 'Massage with Staff A' and the Services would be assigned to the respective Staff Member.