When putting a 3D mesh in front of a grease pencil object, there’s a transparency problem around the mesh. Not sure if it’s alpha aliasing or multiplying or something, but it shows whatever colour is behind the GP object.
Here with a red GP fill, a blue 3D plane and a white background:
Note that if, say, a yellow sphere was to pass behind the GP object, the outline would turn yellow, so I think it’s something to do with the inbuilt grease pencil compositing and transparency.
I was wondering if this is a known bug or a limitation, and also if there’s a known workaround?