Acho que o problema é que eu marquei a tarefa para:
(o) Run whether user is logged on or not
You can specify that a task should run even if the account under which the task is scheduled to run is not logged on when the task is triggered. To do this, select the radio button labeled Run whether user is logged on or not . If this radio button is selected, tasks will not run interactively. To make a task run interactively, select the Run only when user is logged on radio button.
A parte principal disso é essa frase:
To make a task run interactively, select the Run only when user is logged on radio button.
Acredito que o PowerShell precise ser executado interativamente para que o cmdlet Get-CimInstance
seja executado corretamente. Eu mudei minha tarefa para Executar somente quando o usuário está logado e tudo parece estar funcionando agora.