Can't get Append to append a record (association) to a dataset

This is interesting:

If you replace the symbol y with the string "y", Append works fine.

Append[ds, <|"a" -> 5, "b" -> "y"|>]

Also, if you start out with Symbols, then it works fine:

ds = Dataset[{<|"a" -> 1, "b" -> x|>, <|"a" -> 2, "b" -> y|>, <|
    "a" -> 3, "b" -> z|>, <|"a" -> 4, "b" -> x|>}]

Then:

Append[ds, <|"a" -> 5, "b" -> y|>]

Using the Undocumented second argument we can force the Dataset on creation to expect AnyType of type:

Needs["TypeSystem`"];

dst = Dataset[{<|"a" -> 1, "b" -> "x"|>, <|"a" -> 2, "b" -> "y"|>, <|
    "a" -> 3, "b" -> "z"|>, <|"a" -> 4, "b" -> "x"|>}, 
  Vector[Struct[{"a", "b"}, {Atom[Integer], AnyType}]]]

Now Append works even if we feed it a Symbol instead of a String:

Append[dst, <|"a" -> 5, "b" -> y|>]