I think I am missing some information here in regards to your split points. For Bass Clef you are setting to C5, so I am guessing for Treble Clef you are setting to C3. If this is correct I am going to answer based on this information.
1. As a help to see how voices are being recorded into Notion, you can toggle Voice Colors on and off using the shortcut, "SHIFT + Command (CTRL) + V". The voice colors are as follows:
V1 = Black
V2 = Purple
V3 = Pink
V4 = Green
2. When you are real-time recording into the Treble staff and you want to be sure that all input goes into the Treble staff, set the Split point to one note below the lowest note you are going to play. Nothing will go into the Bass Clef unless you cross that point.
3. Make sure you are placing the cursor on the correct staff, whether Treble or Bass Clef before you start recording.
4. Make sure your Input Device is set to Keyboard in the MIDI Record Dialogue box.
When I follow this method, I do not get any crossover to the wrong staff. I can see when Notion changes to the different voices by looking at the Voice colors. I am not a pianist, but I played some random note mess into the score. Here is a picture of what I got in the Treble Clef. You can see that 3 voices were recorded and nothing went into the bass clef.

- Random Multi Voice.png (20.6 KiB) Viewed 5586 times
After that, I went to the bass clef to test and see if there was any input going into the treble clef after setting split point, and to see if there were multiple voices being recorded. This is my bass clef random output.

- Bass Clef.png (37.49 KiB) Viewed 5586 times
Here are the settings in the Record Dialogue that I am using for this test:

- Settings.png (38.83 KiB) Viewed 5586 times
I hope this helps. Let me know if I can be of further assistance.
-Admin