I am having a similar issue that is affecting my whole scene. I am at a dead end and I cannot respect the deadline.
Since I have lots of mirrors I upgraded to 4.12 so that I can use planar reflections in my project. They work well on vertical surfaces. On horizontal surfaces I prefer not to use it since of the problems you mentioned.
All scene is populated with sphere and box reflection. It was looking ok in 4.11
However I am now stuck with weird reflections on all horizontal surfaces. Please check my attachments.
Skylight, Postprocessing all disabled
I have deleted all horizontal planar reflection actors. No result.
I have disabled the planar reflections from the rendering options restarted the project and rebuild all shaders. Reflections bug remain.
What is causing this problem?
