interop between dune and merlin since 2.8 does not work with ppx_expect

Expected Behavior

With the project in https://github.com/ocaml/dune/pull/4478, building with dune and then querying using ocamlmerlin succeeds:

$ dune build
$ ocamlmerlin single errors -filename lib/sublib/bar.ml < lib/sublib/bar.ml
{"class":"return","value":[],"notifications":[],"timing":{"clock":39,"cpu":34,"query":2,"pp":0,"reader":4,"ppx":22,"typer":7,"error":0}}

Actual Behavior

With dune 2.7.1 and merlin 4.1, the actual behavior is as expected. But with later versions of dune, ocamlmerlin cannot find the source file:

$ dune build
$ ocamlmerlin single errors -filename lib/sublib/bar.ml < lib/sublib/bar.ml
{"class":"return","value":[{"start":{"line":0,"col":-1},"end":{"line":0,"col":-1},"type":"typer","sub":[],"valid":true,"message":"I/O error: bar.ml: No such file or\ndirectory"}],"notifications":[],"timing":{"clock":47,"cpu":29,"query":2,"pp":0,"reader":4,"ppx":21,"typer":2,"error":0}}

Reproduction

• PR with a reproducing test: https://github.com/ocaml/dune/pull/4478

Specifications

• Version of dune (output of dune --version): 2.8.5
• Version of ocaml (output of ocamlc --version): 4.11.1
• Operating system (distribution and version): macos 11.2.3