The cmdlet you mentioned should work. However, have a try on listing the existing owner of the group and remove them first
List group owners - az ad group owner list --group MyGroupDisplayName
Remove a group owner - az ad group owner remove --group --owner-object-id
Add a group owner - az ad group owner add --group MyGroupDisplayName --owner-object-id xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
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