Following discussion w/ Jason Grisby at WRI, rather than edit the Q w/ additional detail, the conclusion is that composite `Keys` such as lists of strings, interferes with the above-mentioned named slot access: data = <|"a" -> 1, {"b", "c"} -> 2|> // Dataset; data[#["a"] &] > 1 while data[#"a" &] > Failure[Function, > Association[ > "MessageTemplate" :> MessageName[Function, "slot1"], > "MessageParameters" -> {"a", Slot["a"]& , > TypeSystem`SkeletonForm[ > TypeSystem`Assoc[TypeSystem`AnyType, > TypeSystem`Atom[Integer], 2]]}]]