Hello,
Thank you for the provided details. The first approach should work fine, but it might be required to add the -Server parameter:
Remove-AdmGroupMember -Identity "%sAMAccountName%" -Members "%adm-InitiatorDN%" -confirm:$false -AdaxesService localhost -Server "dc.domain.com"
Also, you can use the below approach that will definitely work.
$Context.TargetObject.Remove("Adaxes://%adm-InitiatorDN%")