"Unknown constructor vendored_dirs" when using OCaml syntax

This dune file works fine:

(vendored_dirs otherlib)

(library (libraries otherlib) (name toto))

This dune file triggers an error:

(* -*- tuareg -*- *)
;; {|(vendored_dirs otherlib)

(library (libraries otherlib) (name toto))|} |>  Jbuild_plugin.V1.send

$ dune build
File "_build/.dune/default/lib/dune", line 1, characters 1-14:
1 | (vendored_dirs otherlib)
     ^^^^^^^^^^^^^
Error: Unknown constructor vendored_dirs

Reproduction

In a fresh directory:

$ cat >dune <<EOF 
(* -*- tuareg -*- *)
;; {|(vendored_dirs otherlib)

(library (libraries otherlib) (name toto))|} |>  Jbuild_plugin.V1.send
EOF
$ dune build

Specifications

• Version of dune (output of dune --version): tested 2.3.1 and latest master e778223b9b1c7f265de978902b97603a0f4bbf24
• Version of ocaml (output of ocamlc --version) 4.08.1
• Operating system (distribution and version): Debian bullseye

Start with the reproduced dune file containing the OCaml syntax extension and run dune build in a fresh directory. Trace why the emitted (vendored_dirs otherlib) is rejected as an unknown constructor, then verify that the reproduction builds without this error.