I ran dune build @fmt --auto-promote and I got the contents of my file that starts with (* -*- tuareg -*- *) deleted.

The logs mention:

File "/Users/anmonteiro/Documents/github/ocaml-h2/lwt-unix/dune", line 1, characters 0-20:
1 | (* -*- tuareg -*- *)
    ^^^^^^^^^^^^^^^^^^^^
Warning: OCaml syntax is not supported, skipping.

Relevant configs:

(using fmt 1.1)
OCamlformat is installed on my switch.