You cannot create paths by yourself, they are essentially the compiled (resolved and unique) variants of Longident.t.
If you are creating a path with a persistent ident, you are essentially asking for the compilation unit m.mli. So the lookup try to read the m.cmi file because it is the path that you asked; and it fails because you are forbidding Env to read cmis with without_cmis.
The functions for finding values by longident are here: https://github.com/ocaml/ocaml/blob/trunk/typing/env.mli#L234