role annotation for newtype (partially?) ignored?

A constraint Coercible o (T a) is ambiguous if the role of a is representational (provided a is not otherwise constrained, of course), but unambiguous if a is a phantom type:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE RoleAnnotations #-}
module CoerceTest where

import Data.Coerce

type role A phantom
data A a = MkA Int

type role B representational
data B a = MkB Int

-- accepted by the type checker (as it should be)
fooA :: Coercible o (A a) => o -> ()
fooA _ = ()

-- rejected by the type checker (as it should be) with
-- "Couldn't match representation of type ‘a0’ with that of ‘a’"
-- fooB :: Coercible o (B a) => o -> ()
-- fooB _ = ()

However, for newtypes something odd happens:

type role C representational
newtype C a = MkC Int

-- accepted by the type checker (but should not be)
fooC :: Coercible o (C a) => o -> ()
fooC _ = ()

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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