Operating system: Windows-10-10.0.18362-SP0 64 Bits
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-05-29 18:08, hash: rB2ee94c954d67
Short description of error
Texture with alpha channel doesn't preview correctly. But Blender 2.79 works fine.
Exact steps for others to reproduce the error
Just load a brush texture with alpha channel and turn on the Alpha option. The texture preview looks incorrect. Whereas the stencil texture looks correct in the 3d viewport.
The texture for test: