Grease pencil question

I have a grease pencil layer that I’m trying to add to. It has a mirror modifier added to it. Just as I start to sketch (see top image) the old grease pencil strokes all disappear and the new one somehow erases the old one. (See bottom image)

After 8 hours of frustration, I think I solved it. Was it because I didn’t have the Additive Drawing Icon checked?