So that can not be the reason then, on the VC where the function does not work, the events are kept forever (no MaxAge enabled).
Do you get any events back when you just call
$entity = Get-Cluster -Name cluster1
$start = (Get-Date).AddDays(-30)
$eventTypes = "DrsVmMigratedEvent","VmMigratedEvent"
Get-VIEventPlus -Entity $entity -Start $start -EventType $eventTypes -Recurse
Do you get any error messages ?
From where do you run this ? From the PowerCLI prompt, from PowerGui,....