'+' in include path with dune

[vzaliva]

I have the following dune file which compiles fine from command line with dune but not under merlin:

(executable
 (name main)
 ; workaround for https://github.com/ocaml/dune/issues/3636
 (flags (:standard -w -27 -I +../cerberus/frontend))) 

In merling-log buffer I see:

"ocaml": {
    "include_dirs": [
      "/home/lord/src/cheri-c-semantics/ubfinder/+../cerberus/frontend"
    ],

If I change it to full path:

(executable
 (name main)
 ; workaround for https://github.com/ocaml/dune/issues/3636
 (flags (:standard -w -27 -I /home/lord/.opam/cerberus/lib/cerberus/frontend))) 

it shows in log:

  "ocaml": {
    "include_dirs": [ "/home/lord/.opam/cerberus/lib/cerberus/frontend" ],

and compiles fine.

It looks like an issue with + handling with -I