brush color defaults to white, if you draw a stroke on vector layer, as soon as you lift off the screen the color being used goes back to white, how do you prevent having to change the color for every stroke?
Is there any way you're activating the Eyedropper (color selection) tool? The gesture for that is a tap-and-hold anywhere on the canvas. And since the primary background color is often white, the brush color will frequently keep changing to white when the Eyedropper is accidentally activated.
Let me know what you think so I can help you get to the bottom of what's happening.