There are two test projects (LM_Test_4.11.2 and LM_Test_4.12.5) contained within this zip file:
https://dl.dropboxusercontent.com/u/15053325/LM_Test.zip
When lighting is built for LM_Test_4.11.2 the lightmaps are baked as expected (as it always has).
However, if I switch (convert) the LM_Test_4.11.2 project to UE4.12.5 and [Build lighting] then the lightmappings is lost.
I also attempted merging the LM_Test_4.11.2 map into a new UE4.12.5 project.
The baked lighting migrates correctly, but again, if I [Build lighting] the lightmapping is lost.
(Fyi, The lightmap coord indices haven’t changed).
Does anyone know how to fix this issue?