@doc "Couldn't find the following modules: Stdlib"

Expected Behavior

Building without errors. 😉

Actual Behavior

Using dune build @all @doc with or without --watch, I almost consistently get errors like this:

File "../../lib_lwt/.gxd_lwt.objs/byte/gxd_lwt.odoc":
Couldn't find the following modules:
  Lwt Lwt_result Stdlib
File "../../lib/.gxd.objs/byte/gxd.odoc":
Couldn't find the following modules:
  Stdlib

These libraries are installed, as the build itself works fine. Even the generated documentation seems okay.

Reproduction

Clearly there's something about my setup that is unique enough to cause this rare issue, but I'm at a loss as to how to narrow things down. I'm hoping you might suggest things to verify or attempt so I can get a bit closer to what you'd need to reproduce this.

Specifications

• Version of dune (output of dune --version): 2.9.1
• Version of ocaml (output of ocamlc --version): 4.13.1 (built with Musl GCC 10.2.1)
• Operating system (distribution and version): Debian GNU/Linux 10 (buster), kernel 4.16.6 on x86_64

Additional information

• Link to gist with verbose output (run dune with the --verbose flag): https://gist.github.com/vphantom/9ccb68c832e39980ec0fa1afa302397a

No source file or test is named. Start by reproducing with dune build @all @doc and compare the --verbose output with the linked gist, using the reported Dune 2.9.1 and OCaml 4.13.1 environment. Done means identifying why documentation generation cannot resolve installed modules and confirming that the command completes without these errors.