This is what happens using the brush. If I zoom in, it disappears, but if I want to draw at a certain zoom, this happens.
What system are you on? Windows or Apple?
Update: It seems that disabling "zoom size with stage" in properties it fixes... But still don't understand why this happens without reason, it's really annoying.
With "zoom size with stage" disable, the grey horizontal lines disappear, but the stage view is just broken. I record this gif... Does someone know how to fix it?