under what circumstances would a renderer's lightmapTilingOffset property fail to update?

I’m loading lightmaps from an asset bundle, and have been doing so successfully for months. Recently they began to look bad and I realized that the lightmapTilingOffset property was inaccurate.

When I load my scene, I assign the correct values to this field, but as of Unity 5.1 the value held by the property was not changing, and was remaining 0,0,0,0 even after the correct value was assigned. I presume the mutator for the property decided not to allow me to make changes to that variable for some reason. Does anyone know what factors could lead to this assignment not executing?

The lightmapIndex parameter updates just fine, it’s only the tiling & offset values that don’t.

Cheers
Jesse

This seems to be related to forward rendering. In deferred mode it works correctly.