Does anyone know why grease pencil layers don't respect 3D space for view order? This is a default 2D animation file with two simple GP drawings, and the layer that is further from the camera shows on top at certain angles. See this screencapture:
OK, after throwing the question out to Twitter, it turns out it is a bug, wherein, (as of v2.91.0) GP objects who's origin points are too close to each other can't seem to sort their display and render orders properly.
The temporary fix is to move the offending layer's origin point further out into the world space. This isn't optimal because now it has a "false" world space coordinate, and now you can't really rotate the GP object properly. But it's a solution for now, as long as your layers rotations don't need to animate.