Is there a nanite bug in 5.3.1? When it's turned off, all meshes revert to their fallback mesh

I’ve got a scene that was originally created in UE4.26. I’ve updated it to 5.3.1 but don’t want to use nanite as the meshes are quite low poly anyway and the nanite overhead actually slows down the scene compared to using the standard meshes. I’ve got nanite unticked in project settings, and the individual meshes have nanite turned off too. The big problem is that I’m finding that the editor is displaying the nanite fallback meshes instead of the standard mesh. And the fallback mesh looks bad because the standard mesh is already quite low poly. Any thoughts? I’m using DC12 and SM6. I’d like to stick with SM6 because I’d quite like to use virtual shadow maps as they’re nice and sharp. Don’t know if that shader model is part of the problem though. Any help or insight welcome.