Unexpected migration error in OCaml 5.3

[johnridesabike]

I just installed a new OPAM switch for OCaml 5.3, and I'm getting the following error when I use the new effect syntax with a file that's preprocessed by the JSOO PPX. I'm not sure where the best place to report this is, so feel free to direct this issue elsewhere.

$ dune build
File "main.ml", line 6, characters 4-25:
6 |   | effect Random_bits, k -> Effect.Deep.continue k (Random.bits ())
        ^^^^^^^^^^^^^^^^^^^^^
Error: migration error: effect pattern is not supported before OCaml 5.03

Minimal example

dune-project

(lang dune 3.17)

dune

(executable
 (name main)
 (modes js)
 (js_of_ocaml
  (flags
   (--enable effects)))
 (preprocess
  (pps js_of_ocaml-ppx)))

main.ml

type _ Effect.t += Random_bits : int Effect.t

let handler f =
  match f () with
  | x -> x
  | effect Random_bits, k -> Effect.Deep.continue k (Random.bits ())

Versions:

$ opam show --field=version ocaml ppxlib js_of_ocaml-ppx
5.3.0
0.34.0
5.9.1

If I remove the preprocess stanza from the dune file, then dune build works with no errors.