HELP NEEDED!!!! Forward Rendering Icons and Material previews have changed - WHY?!

I’m new to UE and am still trying to understand a few (a lot of) things. I expect this is a very basic question, but I’ve just switched a project to forward rendering as it’s a VR job and some, but not all of the material previews, static mesh previews etc are now dark, while some remain as before in deferred rendering. What is the reason for this? Is this telling me something?

Help on this really gratefully received.