At the moment this is intended behavior because if you maintain UVs of a merged actor they will overlap which will generate incorrect results for baked lighting. I do have a workaround if you wish to maintain this listed in public notes.
Basically only lightmap UVs are wiped so you need to assign lightmap index to a throw away channel.
We understand the value of maintaining UVs of merged actors and as far as I know we are looking into solutions for this problem. However, for the time being this is intended behavior.
Well, what you see in the image 3 is one single mesh.
What I don’t understand is why some parts of this has UV map and some other don’t.
Why do the pillars have lost their UV maps while walls didn’t?
We haven’t done extensive testing on this. I haven’t reproduced this with our in-house content. This may be potentially content specific with how you have your meshes setup.
If you would like you could send me a small sample project with just the bugged meshes (before they are merged) and I can look into this. If you want privacy you can PM me here: