The reason for this turned out to be that lightmaps generated in Unity 6 had their type set to "Default" instead of the expected "Lightmap". The reason this happens in the attached repro project is that the project has a TextureImporter preset which has Type=Default. Since this preset has no filters, Unity will apply it whenever a texture is added to the project, including the when lightmap textures are added at the end of a lighting bake. The reason this only happens in Unity 6 is that in prior versions presets were not applied to lightmap textures.