Try setting AFGlobalSettings.ShowExcludedAttributes to true (AFGlobalSettings.ShowExcludedAttributes Property)
[OSIsoft.AF.AFGlobalSettings]::ShowExcludedAttributes = $true
The way excluded attributes are handled by AFSDK depends a lot on how you access them and can be confusing. When ShowExcludedAttributes is false, if you iterate using a foreach loop (which is what powershell does under the hood), excluded attributes will not show up, however using a for loop and the indexer you will get all the attributes. They also appear in the Count an attribute collection. Changing the setting to true means excluded attributes are always iterated over.
That did the trick! I can stop scratching my head.