A little update:
- the more complex the material is, the easier it is to spot this issue (normal map will do the job)
- be sure to use Gear VR specific BP nodes for CPU and GPU throttling and lower GPU modifier to 1 or even 0 - in my case GPU ‘1’ is on the screens, while GPU ‘2’ negates the issue (can be used in Level blueprint)
- material should be directly lit, several intersecting lights is the best way to test, while with indirect lighting issue seems to not be as noticeable
- seems like it has nothing to do neither with texel density and texture size, nor with lightmap resolution