Fails with arrow syntax and line break

Not 100% sure this is a bug - not too familiar with the Arrows lang ext.

{-# LANGUAGE Arrows #-}
{-# LANGUAGE OverloadedStrings #-}

module MyModel where

import Data.Time

personSelect :: Select UserMatchA
personSelect =
  proc () -> row -< ()

outputs

brittany --output-on-errors MyModel.hs
ERROR: brittany pretty printer returned syntactically invalid result.
ERROR: encountered unknown syntactical constructs:
HsProc{} at MyModel.hs:10:3-22
{-# LANGUAGE Arrows #-}
{-# LANGUAGE OverloadedStrings #-}

module MyModel where

import           Data.Time

personSelect :: Select UserMatchA
personSelect = {- BRITTANY ERROR UNHANDLED SYNTACTICAL CONSTRUCT -}

The above fails while if we remove a line break it works:

{-# LANGUAGE Arrows #-}
{-# LANGUAGE OverloadedStrings #-}

module MyModel where

import Data.Time

personSelect :: Select UserMatchA
personSelect = proc () -> row -< ()

Reproduce the failure with the MyModel.hs example using brittany --output-on-errors, comparing the versions with and without the line break. Start by tracing how the formatter handles the HsProc construct; done means the multiline arrow syntax formats without producing syntactically invalid output or an unhandled-construct marker.