I believe the lines that are ignored are more recent than the others.
I was sure the video was supposed to start before i filled, but basically, note how one line looks smaller than the others. It's actually being overlapped by the fill. The unfilled drawing is visible for a split second, but it's a nightmare to pause.
To be specific, I would like to draw only inside a fill.
Edit: I've found alpha lock for levels, but it includes lines, so its not what I'm looking for.
What I mean is how it looks like a string of sausages. I don't have this problem in any drawing program. The video shows me drawing on a vector level, but I also have the same issue on raster levels. It gets better if I turn up smooth, but I don't want to do that, as it completely changes the line when drawing at a small size, especially when drawing a circle.