I'm on 2.93. I take the default cube, delete two faces and then flip the normals.
It looks like this:
I add a blank grease pencil, give it a material and a layer (default values), and then give it a Line Art modifier.
I would expect that all of the edges on the cube would be drawn by the grease pencil, but the end result is this:
If I rotate the cube on the Z Axis, the A edges will eventually flip which of their sides is being drawn, while the B edges will completely disappear. I find this behavior especially confusing because I would assume it should be symmetrical.
It doesn't matter how the Grease Modifier pencil selects edges, all of the methods draw the lines this way. Increasing the line thickness doesn't help, the lines still are drawn only in the same places.
What's going on here? If it's some kind of viewing angle that determines what's drawn, how do I increase it? Why isn't it symmetrical?