I raised the same issue with Freya Holmér, since her Unity Store Asset "Shapes" was displaying the same issue. This was her response:
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this is likely an issue with the way the pixel perfect camera works - so it seems to be a Unity bug yeah :c

it seems to:
• cache the buffer at ResolveColor
• then Shapes draws (Shapes Draw Command)
• then it reapplies the old ResolveColor state in Camera.ImageEffects.DrawDynamic, so anything that was drawn in between gets discarded.

I imagine GL.Lines has the same issue because it happens between these steps too :(

Edit: Actually, this is a specific issue when called at the camera event BeforeImageEffects, if you instead use AfterForwardAlpha it seems to work :)

using( Draw.Command( cam, CameraEvent.AfterForwardAlpha ) ) {
// ...
}

(this is still a Unity bug though, before image effects should absolutely work, but it doesn't)