How to reproduce:
1.Open the repro project "TextureBias" in comment
-> there is only 1 texture file with it's meta file
-> Open the meta file with text editor and note that "mipBias" is -1
2.Select the texture in Unity and observe the Inspector with Debug mode
-> "Mip Bias" on Editor is 0

Expected result: "Mip Bias" on Editor is -1
Actual result: "Mip Bias" on Editor is 0
See attached image in comment.

Note: if "mipBias" is -2 in the meta file, in Editor it will correctly show "Mip Bias" -2

Reproduced in: 2018.2.0a5, 2018.1.0b11, 2017.3.2f1, 2017.2.2p1, 2017.1.3p2, 5.6.5p3

Notes: The texture metadata "Mipmap Importer Test Texture -1.png.meta" file correctly shows mipmap bias as "mipBias: -1". Yet once the texture is imported to Unity the negative bias gets clamped back to 0.

Verified in: 2018.2.0a7