It appears fixity declarations for pattern synonyms only affect modules being compiled in the same run as the module the definition is in. For example, take the following files:

Foo.hs:

{-# LANGUAGE PatternSynonyms #-}
module Foo where

data Type = Type Int RawType

data RawType = Product Type Type
             | Num

pattern a :*: b <- Type _ (Product a b)
infixr 7 :*:

Bar.hs:

{-# LANGUAGE PatternSynonyms #-}
module Main where

import Foo

value = Type 0 $ Product (Type 1 Num) $ Type 2 $ Product (Type 3 Num) $ Type 4 Num

somethingElse = 23

main = case value of
           _ :*: _ :*: _ -> putStrLn "Success"
           _             -> putStrLn "Fail"

On the first compile, the executable will print "Success". Modifying Bar and recompiling will result in it printing "Fail". Modifying both and recompiling results in "Success" again.