I have figured out what the problem is. In case someone facing the same issue, I post what I have found here.
I was inspired by this thread (Android Landscape texture). On android, multi-layer landscape material is not supported (so landscape painting is impossible). It is neccessary to write everything into a single layer of material.
The picture below is UE’s vehicle template with a singlelayer material from Epic’s “Soul City”. It looks pretty good on my cellphone.
You can rewrite your painting logic into your landscape material (instead of painting it manually); or (if it is impossible) use a staticmesh as replacement.