This should be working correctly, I guess there is something wrong in your setup.
What you see in your source map is coherent with what you see in the grid: the default cortex (selected in the green in the database explorer) does not include the cerebellum.
- Make sure your edit cortex (cortex+subcortical structures) is selected and displayed in green in the anatomy of your subject,
- Restart Brainstorm (to make sure no residual interpolation matrix remains in memory)
- Compute again the forward and inverse models.