Strange missing `system-msvc` dependency in chain on non-Windows target

[gr1mpatr0n]

When building a simple project with the "Set up OCaml" Github Action, and using ocaml-option-no-compression in my Dune dependencies, I am getting the following error:

Run opam install . --deps-only --with-test
[ERROR] Package conflict!
  * Incompatible packages:
    - (invariant) → ocaml-base-compiler = 5.3.0
    - deps-of-gnuplotscatter → ocaml-option-no-compression → ocaml-variants >= 5.1.0~
No solution found, exiting
    You can temporarily relax the switch invariant with `--update-invariant'
  * Missing dependency:
    - deps-of-gnuplotscatter → ocaml-option-no-compression → ocaml-variants >= 5.1.0~ → system-msvc
    unmet availability conditions: 'os = "win32"'

Error: Process completed with exit code 20.

This seems to suggest that system-msvc is a missing dependency, but I am not even building on or for Windows - this is for Ubuntu and macOS only.