I have seen FSDb use cracks in the asphalt in the middle of an unmarked road to represent a double yellow line in the visualization. Very reproducible on a particularly cracked road in my neighborhood.
I have the opposite problem. For smaller / neighborhood two lane two way roads around here, it's common to not paint any middle stripe, and the black tar line running down the middle is the line, which it should be looking for and visualizing like a stripe. FSDb is only figuring this out like half the time, the rest of the time it acts like it's on some kind of unmarked one way road and tries to ride the middle.