This behavior is usually caused by the Line Smoothing setting in the Tool Wheel. If smoothing is set to 100%, any stroke you draw will automatically snap into a perfectly straight line between its start and end points.

How to Adjust Line Smoothing

  1. Tap to open the inner ring of the Tool Wheel.
  2. Tap the squiggly line icon to reveal the smoothing slider.
  3. Adjust the slider to a lower percentage. A value between zero and 30% is common for drawing.

Pro Tip: You can customize the preset smoothing values with a Tap+Hold gesture.

Why the Fill Tool Stroke Vanishes

The Fill tool creates a filled shape between the start and end points of your stroke. If smoothing is at 100%, the stroke straightens into a line with no area to fill, causing it to disappear as soon as you lift your stylus.