There is a defect causing this issue fixed in v8.7
Running a Update Server Properties job after the failed Agent Install Job for the most part re-establishes communications to the clients.
I've found that manual intervention is needed in some cases as often the agent fails to restart after the upgrade. Restarting manually never seems to fail though.
Does this happen for agents on all operating systems or only on specific Operating Systems ?
Out of 3 platforms tested so far, Windows, Linux and AIX, only AIX servers have issues.