this happened two days ago while I was animating. My brush strokes are doing this really annoying thing where instead of painting a solid black line they paint a transparent stroke with very small horizontal line patterns in them while my pen is pressed, then when I pick up my pen from my cintiq the solidify. I don't know how to get my original solid brush strokes back, I almost never on forums before but I haven't found a solution to this problem anywhere!
I guess you might have enabled Pressure or Tilt accidently. Try disabling it from the bottom of tools panel and then draw.
Also please confirm if the problem persists even after you quit and relaunch Animate.
In case this doesn't help, can you share a video recording of your screen to show the problem.
It sounds like you're using the Paint Brush Tool as opposed to the Brush Tool. The Paint Brush Tool can apply vector art to the brush strokes. You will see those lines with the Paint Brush but not the regular Brush.
I need to see a screenshot if possible - sounds like a video card related issue - what kind of system are you on?