Thanks for your report, we don't receive similar issues, it would be better if you can peovide a minimal code to reproduce this problem. 
Hello. I was not able to reproduce our problem with LOD bias in an artificial repro example - LOD bias works as it should.
But there is still some significant difference in how textures are sampled on RX5700 compared to previous generations and Nvidia cards which breaks our CI apart. I understand that such changes may happen between GPUs generation, but it would be good to know if what I see is expected behavior or a bug, and a hint what could be tweaked to get results almost equal to reference as we have with all the other GPUs here. Let me demonstrate my findings.
Sample scene looks as follows:
First: mipmap level selection is pretty different from what I see on the other GPUs. Mipmap level queried via textureQueryLod(), encoded as a color: white for level <1.0, red for levels between 1.0 and 2.0, green for 2.0 <= level < 3.0, etc.
"All other" GPUs:
RX 5700 XT:
I understanda that mipmap level selection is not standardized, but combined with the next finding sampling in a different manner on a different mipmap level gives a good visible diff.
This diff between pre-RX5700 and RX5700 XT renderings shows some radial pattern, even in the areas where mipmap levels are equal betwenn RX5700 and non-RX5700:
Interresting that this difference is not present on the planes orthogonal to the camera view direction - look that there is almost no diff pixels on the far plane orthogonal to the camera view, whereas plane with a slope has significant differences texture sampling:
It would be great to have comments on if this behavior is expected or not.
Please let me know if you need more materials.
Sounds like a bug which only RX5700 has, could you please provide me a minimal code which can reproduce this issue? 
Thanks very much!!
