`Error: Unbound module Foo` with one switch, no errors with different switch

By happenstance, I'd been using ocaml-variants.4.07.1+flambda since before I started using this plugin, and all was well, until I started using ocaml-base-compiler.4.07.1 (i.e. the "regular" switch?) earlier this week. After that change, any references to modules defined within my working directories fails with a squiggly popup reading Error: Unbound module $module_name.

Immediately around the error in the log is this:

<< Making lastDefinitions with type error for file:///home/chas/dev/project/test/testgeom.ml
type error here File "/home/chas/dev/project/_build/.lsp/testgeom__Testgeom.ast", line 1, characters 5-9:
Error: Unbound module Geom
Sending notification {"jsonrpc": "2.0", "method": "textDocument/publishDiagnostics", "params": {"uri": "file:///home/chas/dev/project/test/testgeom.ml", "diagnostics": [{"range": {"start": {"line": 0, "character": 5}, "end": {"line": 0, "character": 9}}, "message": "Error: Unbound module Geom", "severity": 1}]}}

Nothing spectacularly revealing. AFAICT, it's the same message produced by typing something that really doesn't exist open Bzzzzt.

The workaround for me currently is to just use the flambda switch. Perfectly fine of course, but (a) it seems like there's an actual problem here, and (b) I'm actually not using flambda in CI/production because the programs in question actually are faster without it. 🤷‍♂️

I can reproduce the problem at will with just:

opam switch 4.07
eval $(opam env)
rm -rf _build; dune runtest

…and then restarting the RLS to pick up the .merlin changes.

Riproduci con i comandi indicati opam switch, eval, dune runtest e RLS restart, quindi esamina le modifiche generate a .merlin e il log di RLS nell'area relativa a testgeom.ml. Confronta la risoluzione dei moduli con gli switch regular e flambda. È completato quando i moduli della directory di lavoro vengono risolti con ocaml-base-compiler.4.07.1 senza la diagnostica di modulo non associato.