Hello,
I can reproduce the problem with 100% success rate 
. The assets I uploaded were made from scratch as I couldn’t find (probably didn’t save) the source file from my initial bug report. Now I tried again, I was especially careful not to do anything atypical and got the same problem. Here are the simplest steps I came up with that still lead to the bug:
- create cylinder and just to be sure turn in to editable poly
- select all the polys of the cylinder, assign material id1, select top half of the cylinder polys and assign material id2
- create multimaterial with two submaterials and assign it to the cylinder
- create two bones inside the cylinder
- assign a skin modifier to the cylinder, and use the two create bones for skinning (no need to tweak the skin, default will do)
- export the cylinder with bones
- in UE, create the simplest possible material, enable PN triangle tessellation and turn on crack free feature and assign it to the cylinder into both material slots
That should be all. Let me know if and how can I help some more.
Thanks.