As it currently is in lw, lw 2018, when an object is saved from 2015, will load the 2015 surface.... when you save the object from 2018, the lw2015 surface is destroyed, so when you load that object back into 2015, the surfaces are gone.

That is lw default behavior. My plugin, allows you to preserve that information, and, depending on which app you load it in, will load the correct surface (either the one that was saved in 2018, or the one that was saved in 2015.)