Hi @cliff and welcome to the forum!
It’s a bit unfortunate that the .net/python gets confused with this…
I gave an answer here: Reading multiple test step outputs (python) - #8 by jb111
Basically you need to get C# to give you the concrete type instead of an interface type ITestStep.
parent = self.GetParent[MyParentType]()
I will try to investigate if this is something we can fix. But for now you can use this.