Consider using root rescript.json if nearest one does not have a setting.

If I have a monorepo:

.
├── common
│   ├── src
│   └── rescript.json
├── app
│   ├── src
│   └── rescript.json
└── rescript.json

and, in the root rescript.json, I set the suffix to something that is not the default, e.g.

{
  "package-specs": [
    {
      "module": "esmodule",
      "in-source": true,
      "suffix": ".res.mjs"
    }
  ],
}

the suffix resolution for opening the compiled JS file in both the common and app subprojects does not work, since the default is .js. One would need to add the suffix in the subproject's rescript.json as well.

The order of evaluation should be
local > root > default fallback

Start by tracing the suffix resolution used when opening compiled JavaScript files in the common and app subprojects. Check how local and root rescript.json files are discovered, then verify that local settings take precedence over root settings and that defaults are used only when neither defines the setting.