Refer below issue,
RFE is raised with BMC team for review. This is a bigger gap in Event Management flow and hope the Product team provides a resolution soon.
The issue is actually little different here (Instead of Patrol agents, mcxp instances are restarted) however I am suspecting mcxp also behaves the same way but we are not sure.
To be sure on this, We are trying to reproduce the issue in our DEV environment and suspecting role of HA cell here, i.e old events that might have got stuck (due to sync issues between Primary & Secondary) on the secondary cell got triggered once the activity was changed from "Stand-by" to "Full".
When Patrol Agents are connected to mcxp in Site 1, it would have triggered all OPEN events to the cell specified in configuration.
As you know, BIIP3 do not support application level HA. Hence, in this case, when Agents are added to mcxp in Site 2 and have OPEN events, it will re-trigger the events to mcxp instance and in turn create events in cell.
This is my understanding of BiiP3 out of box behaviour and you can get it confirmed with BMC Support team.