The docstring for coreExpandTyCon_maybe says it should do something different than tcExpandTyCon_maybe, but it doesn't:

-- ^ Used to create the view the /typechecker/ has on 'TyCon's.
-- We expand (closed) synonyms only, cf. 'coreExpandTyCon_maybe'
tcExpandTyCon_maybe = ...

-- ^ Used to create the view /Core/ has on 'TyCon's. We expand
-- not only closed synonyms like 'tcExpandTyCon_maybe',
-- but also non-recursive @newtype@s
coreExpandTyCon_maybe tycon tys = tcExpandTyCon_maybe tycon tys

If the code is correct, and the comment is just outdated, than Note [Newtype eta] and the docstrings for Type.coreView and Type.tcView should be updated as well.

The last big change to coreExpandTyCon_maybe was made in commit fdf86568.