This looks like an application bug to me - here's what the game does with the
minimap fog of war in the apitrace (right at the start of each frame, so it's
easy to find):

1. Uploads the fog of war to GL_ALPHA texture 2487
2. Binds framebuffer 2
3. Attaches texture 6613 as the color0 buffer
4. Clears color to (0, 0, 0, 0)
5. Unbinds framebuffer 2
6. Binds framebuffer 2
7. Attaches texture 6613 as the color0 buffer
8. Binds 6613 as texture0
9. Binds 2487 as texture1
10. Draws a full-screen quad with a shader that boils down to
  a = pow(texture1.a, 8);
  color0 = a * (0, 0, 0, 1) + (1 - a) * texture0;

Reading from and writing to the same texture in this way is undefined without
inserting the appropriate memory barriers.