dataset - DeleteMissing level spec confusion

Given:

titanic = ExampleData[{"Dataset", "Titanic"}]

Why does this correctly delete Missing[] elements:

 titanic[GroupBy@Key@"class", DeleteMissing[#, 1] &, "age"] // Normal

but not the default level spec? Seems to violate key transparency in Associations/Datasets.

 titanic[GroupBy@Key@"class", DeleteMissing, "age"] // Normal


(* <|"1st" -> {29, 1, 2, 30, 25, 48, 63, 39, 53, 71, 47, 18, 24, 26, 80, 
   Missing[], 24, 50, 32,... |>  *)

DeleteMissing also works on the bare list, as would any function applied at that Dataset slot. On the other hand:

<| "key" -> {29, 1, 2, 30, 25, 48, 63, 39, 53, 71, 47, 18, 24, 26, 80, 
   Missing[], 24, ... |> // DeleteMissing[#, 2] &

Level 2 is the minimum needed to delete missing element?

Answer

UPDATE The described behaviour is not a bug as DeleteMissing is explicitly listed as a descending operator in the documentation. See the response by @TaliesinBeynon.

I will delete this response after the "accept" has been transferred.

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Original Response - WARNING: The following analysis is incorrect.

This is yet another manifestation of the WRI-confirmed bug encountered here and here.

Correct compilation, with DeleteMissing applied at the right time and level:

Dataset`CompileQuery @ Query[GroupBy@Key@"class", DeleteMissing[#,1]&, "age"]

(* Dataset`WithOverrides@*GeneralUtilities`Checked[
     GroupBy[Key[class]] /*
     Map[Map[GeneralUtilities`Slice[age]] /* (DeleteMissing[#1,1]&)]

   , Identity
   ] *)

Incorrect compilation, with DeleteMissing applied too early:

Dataset`CompileQuery @ Query[GroupBy@Key@"class", DeleteMissing, "age"]

(* Dataset`WithOverrides@*GeneralUtilities`Checked[
     GroupBy[Key[class]] /* 
     Map[DeleteMissing /* Map[GeneralUtilities`Slice[age]]]
   , Identity

   ] *)