Visual Studio has a concept called data visualization that is used to view values in the debugger. Each data type has a visualizer, and you can actually create your own visualizers if you want to display a type's data differently.
Sometimes visualizers have a hard time displaying data - especially when collections, generics and inheritance are involved. That's what you are seeing here, is the default visualizer simply getting confused by the combination of generics and inheritance used to create the collection.
It doesn't mean there's anything wrong with the collection - you are just hitting a limitation of the visualizer in VS.
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