Static/Stationary Lightmaps and any Shadow Map that isn’t the closest to the light source are not rendered in the Planar Reflection with 4.15. This means that mirrors for example will show a fully unshadowed version of the player and environment, and that water using planar reflections is showing a partially unlit scene, completely breaking the artistic style intended.
Screen Space Reflections are the only ones actually showing the correct shadows, but they do not work together with Planar Reflections, as they have lower priority than Planar Reflections.
sorry for the late reply, it seems that this issue is irreducible in a sample project. I do get the issue in my own project but the Terrain material there has 35 layers, each of which has 5 textures using the shared: wrap sampler and 10 global samplers which essentially make it 13 texture samplers. Is this perhaps a limitation of the planar reflection renderer?
Edit: Nevermind it happens even with a sample material there that only has one layer. This is odd, I wonder what broke it.
That is quite odd indeed. I am really not sure what could be happening, but one thing to try would be to delete your Saved and Intermediate folders. Perhaps those are holding on to some dirtied files and settings which are sticking around and causing the error. Be sure to make a copy before deleting those files however.
Let me know the outcome and if you need further assistance.