How does GADTs affect type inference in this case?

Say I have a monad that emits soft failure through Writer. A simplified version goes like this (for the record, I'm using GHC 9.0.2):

{-# LANGUAGE BlockArguments #-}{-# LANGUAGE FlexibleContexts #-}-- {-# LANGUAGE GADTs #-}{-# LANGUAGE ScopedTypeVariables #-}module Lib whereimport Control.Monad.Writer.CPSimport Data.Maybeverify :: MonadWriter [String] m => some -> args -> m ()verify _ _ = fix \(_ :: m ()) ->  do    let warn = tell . (: [])        a = Just 1 :: Maybe Int        b = Just [1, 23] :: Maybe [Int]        c = Just (id :: forall a. a -> a)        -- isJust' :: String -> Maybe a -> m ()        isJust' tag v = unless (isJust v) do          warn $ tag <> " is Nothing"    isJust'"a" a    isJust'"b" b    isJust'"c" c

All goes fine until I add GADTs extension, then GHC fails to find the most general type for isJust':

• Couldn't match type ‘[Int]’ with ‘Int’      Expected: Maybe Int        Actual: Maybe [Int]• In the second argument of ‘isJust'’, namely ‘b’      In a stmt of a 'do' block: isJust'"b" b      In the expression:        do let warn = tell . (: [])               a = ...               ....           isJust'"a" a           isJust'"b" b           isJust'"c" c   |22 |     isJust'"b" b   |                 ^/var/tmp/demo/src/Lib.hs:23:17: error:• Couldn't match type ‘a0 -> a0’ with ‘Int’      Expected: Maybe Int        Actual: Maybe (a0 -> a0)• In the second argument of ‘isJust'’, namely ‘c’      In a stmt of a 'do' block: isJust'"c" c      In the expression:        do let warn = tell . (: [])               a = ...               ....           isJust'"a" a           isJust'"b" b           isJust'"c" c   |23 |     isJust'"c" c   |                 ^

At which point I have to uncomment type annotation for isJust' to get this to type check - I'm wondering what is at play here, I don't think I'm using any GADTs features?

Side note 1: I usually just slam a NoMonomorphismRestriction to see if it helps, it didn't.

Side note 2: That use of fix is just for getting a hold on that m in the presence of ScopedTypeVariables - a side question is whether there are better ways to do this without relying on verify having an explicit type signature (say this function is defined inside an instance).