Whenever notes have been selected for a particular clef, a badge appears on the clef showing the total number of notes selected in that staff.
Remember- For any given staff, there might be selected notes that aren’t currently visible on the screen.
To be sure you are clearing (de-selecting) all the notes for a clef…
- Tap the green clef badge showing the number of notes currently selected.
- After it turns into a red X, tap it again to clear all of the notes in that staff