Steps to reproduce:
-Open the attached scene (ivy_files)
-Select the Particle System in the Hierarchy Tab
-Simulate the system in the scene view or press play
-Note that the lower diagonal half (bottom right tri) of the billboard is black
-Go to Particle Material in the Project Tab
-Remove the Normal Map - problem solved (but no more 3d feels)
-Add the Normal Map again
-Increase the Tiling parameter in Main Maps
-Notice that regardless of position of the image maps, the whole lower right tri remains black

Alternative reproduction steps:
1) Open attached project "TangentBug
2) Open and play scene "Test"
--- You should see a billboard system on the left and a mesh one on the right with shader that shows tangents applied