If it is happening to disconnect the ESXi hosts for 5.5 and 6.5, then I would strongly advise to check the port 902 UDP if it is open between ESXi and vCenter. port 902 UDP is used for heartbeat. If the heartbeat is missed or not received within the time (Default of 60 seconds, which you changed to 120 with the advanced parameter), the ESXi will be marked not responding.
VMware Knowledge Base
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