I wonder if it’s simply you’re not understanding how the tool works. It doesn’t work the same as Bézier curves in Adobe Illustrator or Affinity Designer. If you just tap tap tap, you will get straight lines.
You should tap once to create a starting point then tap press and drag to get your first control point (no handles) you’ll see a blink of color to let you know you made a control,point, then with the pencil still on the screen, drag. This will change the line to a curve.