And there it is.
You cannot record SI by playing the bass with your mouse.
What you are seeing is not an input reading, but an output reading.
SI and other synths provide triggers, such as onscreen keyboards in the GUI, for auditioning purposes.
These trigger the synth directly, and bypass the MIDI track, which is where the data actually needs to be recorded.
You can record to the associated MIDI track with an external controller.
Or, you can enter notes in the PRV.
The preset rhythm patterns can be dragged to your MIDI track, and will play back.
With SI, or any soft synth, you don't record the sound. You record the MIDI data that triggers the sound.
This is a notoriously confusing concept for new users to grasp.
If you insert the synth by selecting MIDI Source Track and First Synth Audio Output, instead of Simple Instrument Track, it is a little easier to understand what is really happening.