This seems to be a bug in WFA w.r.t. this Cmdlet Add-WfaWorkflowParameter. It doesn't happen all the time but once in a while, all or some of the added parameters are not returned at the end of execution. I've been able to reproduce the issue locally.
I'll log a bug with WFA. You can raise a customer case with NetApp to track its progress.
Your inconvinience is regretted.
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