What values get unwrapped from a Monad with do-notation and what determines this?

Question

I am still a bit new to Monads and have just realized that something I thought I understood can't be the case. In the MVE that follows I have a hard time understanding why next <- scanChar can return just a single Char and bind it to the next. Wouldn't it rather be [(Char, Int, d)] that would be bound to next since this is what is unwrapped when using do notation given that all of the value within the Monad will be unpacked, or is there something in the constructors or instances that will make it otherwhise?

import qualified Data.Set as S

import Control.Monad

type CharSet = S.Set Char

data RE =
    RClass Bool CharSet

newtype RegModule d a =
  RegModule {runRegModule :: String -> Int -> d -> [(a, Int, d)]}

instance Monad (RegModule d) where
  return a = RegModule (\_s _i d -> return (a, 0, d))
  m >>= f =
    RegModule (\s i d -> do (a, j, d') <- runRegModule m s i d
                            (b, j', d'') <- runRegModule (f a) s (i + j) d'
                            return (b, j + j', d''))

instance Functor (RegModule d) where fmap = liftM
instance Applicative (RegModule d) where pure = return; (<*>) = ap

scanChar :: RegModule d Char
scanChar = RegModule (\s i d ->
  case drop i s of
    (c:cs) -> return (c, 1, d)
    [] -> []
  )

regfail :: RegModule d a
regfail = RegModule (\_s _i d -> []
                )
regEX :: RE -> RegModule [String] ()
regEX (RClass b cs) = do
  next <- scanChar  
  if (S.member next cs)
    then return ()
    else regfail
 
runRegModuleThrice :: RegModule d a -> String -> Int -> d -> [(a, Int, d)]
runRegModuleThrice matcher input startPos state =
  let (result1, pos1, newState1) = head $ runRegModule matcher input startPos state
      (result2, pos2, newState2) = head $ runRegModule matcher input pos1 newState1
      (result3, pos3, newState3) = head $ runRegModule matcher input pos2 newState2
  in [(result1, pos1, newState1), (result2, pos2, newState2), (result3, pos3, newState3)]