Recursion tactic doesn't fire when deriving mutual recursion

[isovector]

From AutoThetaFix.hs test:

data Fix f a = Fix (f (Fix f a))

instance ( Functor f
           -- FIXME(sandy): Unfortunately, the recursion tactic fails to fire
           -- on this case. By explicitly adding the @Functor (Fix f)@
           -- dictionary, we can get Wingman to generate the right definition.
         , Functor (Fix f)
         ) => Functor (Fix f) where
  fmap = _

Note the FIXME. Recursion should be able to handle this just fine, but it doesn't! Unclear why, but I intend to find out.