If you’re purchasing assets from any source other than the UE4 marketplace you will more than likely have to deal with this as most model sites do not have standards set for requirements for being “game ready.” You will need to clean up the UVs to account for this. Exporting as FBX 2013 vs 2014 you will not see a difference for the UVs.
The reason it worked before with the generated UVs was that there were two methods. Now there is only one within UE4.
The first method would be to unwrap the model, but it wouldn’t cut seams or do so right. This would result in stretched UVs and artifacts for the Lightmap. The second method (the one now used in 4.5) is to base the UV off the UV separation from the Texture UV (UV0) and repack them. If there isn’t a good setup it will still look bad which is probably the problem you’re seeing with yours.
Tim