Joining objects breaks some 3D Scene Textures

I have six plane objects, each UV-mapped, each with unique materials and textures. They are working fine as separate objects, and they form a real-time cube map. If I Join them into one object however, some of the faces stop displaying their 3D Scene Texture. (The purpose of joining would be to eliminate the seams that are visible at the object edges.)

As far as I know, there is no limitation for how many 3D Scene Textures can be on a single object (is there?)… so I thought maybe this is a bug.

Here is the .blend with the objects still separate. You can test before joining that it is working in the browser - then press Ctrl J and test again to find that it's broken.

It's not a bug. If you open console you can see a warning that cause a problem. When you joined the planes the UV-maps with different names are not joined together and because of Blend4Web supports only two uv-maps on object the rest was discarded.

D'oh! Sorry, false alarm!
Today I learned: Always check the console before asking if it's a bug.

Today I learned: Always check the console before asking if it's a bug.

Yes, it's very helpful in 90% of all problems.