Open gives "Unbound Module" error unless module is compiled?

[liffiton]

I've run into an issue, but quite possibly I'm just missing something and it's supposed to be this way.

Specifically, if I have first.ml:

let x = "Hi"

And second.ml:

open First
let _ = print_endline x

Then ocaml-lsp gives me an "Unbound module" error on the open line in second.ml. If I compile First using ocamlc -c first.ml, producing first.cmo and first.cmi, then the error goes away.

I'd expect ocaml-lsp to be able to find the module whether it's compiled or not, because the correctness of the code doesn't depend on that first module being compiled. Is that right? Or am I missing something about how all of this is supposed to work?

Thanks for any help or advice.