Jsoo sourcemap and recent versions of dune

I am coming back to some jsoo project someone else had been working on, and I notice that with the (presumably) newer versions of dune and/or jsoo I now have installed, the sourcemap functionality is no longer working in the firefox debugger. Would someone know which combination of settings is supposed to make it work? I have a suspicion it is related to the map_workspace_root dune setting but neither value makes it work.

I am using dune 3.7.1 with ocaml 4.14.0 and js_of_ocaml 5.0.1, and the following in my dune file:

 (dev (js_of_ocaml
  (compilation_mode whole_program)
  (flags :standard --target-env browser --file something))))