Have you tried removing the Contact Selection constraint?
If I remember correctly from when I set up PD integration in my test environment, PD notification in Checkmk 2+ is not associated with Checkmk users. Rather, PD integration is controlled by the Integration Key, which is linked to a specific service in PD.
This means that a Checkmk rule that defines PD integration says “Tell PagerDuty that Service1 is down.” PD then decides who to notify based on its configuration. This is fundamentally different from Checkmk rules that define which Checkmk user(s) to notify of an event.
Hope this helps,
Jason