`dune build @check` should pass if `dune build @all` does

I'm working on improving the build system in ahrefs' monorepo. Part of this work is allowing to build as many things as possible in one command, and to speed up the feedback loop from dune. dune build @all tends to be pretty slow, even in subparts of the monorepo. So I wanted to use dune build @check instead, at least during development. But there are directories where dune build @check doesn't pass, while building the @all alias is successful. It feels like a bug, given that @check should be a subset of @all.

Start by reproducing the discrepancy in the affected monorepo directories with dune build @check and dune build @all, comparing which targets or errors differ. The work is done when @check succeeds wherever the corresponding @all build succeeds, with regression coverage for the identified case.