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
- In the center of the Tool Wheel, Tap the squiggly line icon to reveal the smoothing slider.
- 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.