Base schema (schema extensions) and Schema Stitching project not working

I'm having trouble using the Base Schema/Schema Exension part of Hive, with a **Schema Stitching** project

I want to use it to define some custom directives, as well as some shared scalars and types.
As an example, I add this to the Base Schema and hit save:
```
directive @mycustom on FIELD_DEFINITION
scalar DateTime
```

With a schema like (the rest omitted for brevity):
```
type User {
id: ID!
created: DateTime!
name: String! @mycustom
}
```

I get errors both from `hive schema:check` and `hive schema:publish` :

```
✖ Detected 2 errors

- Unknown directive @mycustom.
- Unknown type DateTime.
```

If I create a **Monolithic Schema** project, I can **publish just fine** with the same inputs. `hive schema:check` does however give me the same error in both project types.

Am I doing something wrong?