Issue is that due to the strange definition of render in grease pencil (meaning should be rendered similar to rendering). This included normal viewport rendering in OB_RENDER and OpenGL render in OB_RENDER. For other rendering modes the overlay vertex opacity would be used. This patch sets this value to 1 when rendering via a scene strip override. NOTE: that this isn't a good solution as I expect that users want to use the opacity of the Grease pencil object. Perhaps the GPencil team has a better solution for it.