The source of the error appears to be a stray DefaultValue="None" that's contained within the inner "Test_Nested_TestContainer" split pin (copy/paste the node to notepad to see this). If you refresh the node, then the DefaultValue="None" goes away. Additionally, it gains a tooltip entry.
There appears to be a discrepancy with how pins are created during a split when performed manually vs refresh.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-161254 in the post.