To reproduce:
1. Open new project
2. In Hierarchy window, create a new sprite
3. In Inspector window, add a sprite to Sprite property (default ones will do)
4. Enable wireframe mode in scene window
5. Rotate selected sprite around

Expected: sprite's wireframe is always visible
Actual: sprite's wireframe is not visible when facing away from viewport camera (check .gif attached)

Reproduced in: 5.4.6f1, 5.5.4p2, 5.6.2p4, 2017.1.0f3, 2017.1.0p1, 2017.2.0b4, 2017.3.0a1

Note: since sprites are two sided, wireframe should also disregard to what side is sprite flipped (or rotated)