It’s expected to work this way. From this documentation:
Note: When a base path is specified as a form argument, the form renderer assumes all fields on the form are accessed through that base path, so all attributes to be included in the form or returned from it must be included in the model.
You can just continue to reference the formModel throughout the workflow, that is kind of the point of it to make the passing back and fourth of form variables easier to access from a single map rather than individual return attributes.