Bug: "ast changed" error with certain infix operators

**Describe the bug**
```
ocamlformat: Cannot process "ast_changed.ml".
  Please report this bug at https://github.com/ocaml-ppx/ocamlformat/issues.
  BUG: ast changed.
```

**How to Reproduce**
`ocamlformat --profile=janestreet --enable-outside-detected-project --disable-conf-files ast_changed.ml`
produces:
```
(x.@(y) <- z) @ []
```
