In 5.x we no longer run the same unwrapper DLL on Windows and Mac. This means that when GI data is built on one platform with a certain set of lightmapping UVs (the realtime GI UVs are based on those, and the baked lightmaps use them too) the UVs produced on another platform during mesh import can be different.
The resulting mesh can thus have a different vertex count.

This is a regression compared to late 4.x.

This is a problem when loading the data built on one platform on another platform the UVs will not match with the output and one will see garbled output in the scene.

The realtime GI UV's patched from the snapshot can have a different count than the vertex count of the mesh and that leads to errors when batching:
"Failed setting triangles. Some indices are referencing out of bounds vertices. IndexCount: XXXX, VertexCount: YYYY"

Currently the only workaround is to rebuild lighting on each Editor platform.