Reproduction steps:
1. Open the reproduction project
2. Open mainScene.unity
3. Press Play

After hitting play you can see the 2 terrains that were loaded in, are sampling from the lightmap meant for the cube in the mainScene, which is of index 0. As opposed to index 1 and 2 which are the proper lightmaps for these terrains.

Image of 5.0.2p4 https://i.gyazo.com/256c12381abef3b2c1ee4cfb4ad1039f.png
Image of 5.1.3p3 https://i.gyazo.com/9c4cf6659416c816939fee4eadeea6bc.png
Image of 5.2.1p2 https://i.gyazo.com/4a2734cb5d557629e8067ec5b999f9d5.png

note: the Sphere loaded in from terrain02.untiy correctly sampling from lightmapIndex 2 which proves this works properly for mesh renderers but not terrains.