Tested Standard (Specular setup) shaders with avatars and world. It seems that this shader auto replaced by Standard shader. As result, Specular texture doesn't work, transparent and cutout don't work too. Test world ID: wrld_06061ec1-0fc3-4d0b-ba82-8f740aece015

The only difference between spheres - left set using Standard and right set - Standard (Specular setup) with specular map attached.

Comparsion screenshots included.