Blocky realtime lightmaps - uneven

Hi,

I am getting very blocky realtime lightmapping on the floor of my model. See attached.

Both objects set to static, both unity primitives. Light material set to realtime, for some reason the floor UV lightmaps just behave strangely. i recreated these materials and settings in a different scene and lighting looked fine and even.

Cleared all lighting data, rebuilt… same problem

Any ideas why this is happening? Using Unity 2017

select the .fbx model → generate lightmap uvs (in the fbx import settings)