Yep, all the texture samples should use mipbias.
With texture streaming, the idea is to conserve VRAM by creating a bunch of resized copies of the full resolution texture. So for a 512x512 texture, the engine precomputes a 256x256 version, 128x128, all the way down to 1x1. The engine labels these images 0, 1, 2 … (mip level)
When the texture is really far away, the full resolution 512x512 one is not taking up heaps of VRAM, the engine just uses mips 7 to 9.
The way the engine calculates what mip level to use is a black box to me, but it is based on distance.
With mipbias, after the engine is done saying ‘use mips 7-9 of this texture,’ you manually override it and say ‘no, subtract 1 so it looks clearer.’
If every single material in your game needs mipbias, there is probably a .ini setting somewhere that changes the way mips are calculated.