Menhir formatting broken during dune watch mode

[amongonz]

When VS Code triggers a Format Document command on a Menhir file (either by user request or format-on-save) and dune is running in the background in watch mode (dune build -w), the entire file is split into a line per token:

%token

End_of_input

%token

Dot

Colon

%token

<int64>

Integer_lit

// ... The entire file like this.

When not running in watch mode, the format command doesn't do anything, which I understand is the intended behaviour.

I reproduce this on vscode-ocaml-platform v1.12.2, ocaml-lsp-server v1.16.2, dune 3.9.0 and ocaml 4.14.1.