To reproduce:
1. Open attached project;
2. Open scene 'scene";
3. Select Game Object "animation/sprite" from Hierarchy;
4. Go to Animation Tab;
5. Move red cursor to desired time;
6. Change the sprite via Sprite Renderer Component;
7. Once again move red cursor again to yet another time;
8. Once again change the sprite via Sprite Renderer Component;
9. Move red cursor back and forth over key-frames;

Actual result: All key-frames show same sprite (the last one you have chosen) as you move red cursor over them.
Expected result: All key-frames show different sprite when red cursor is being moved.

Reproduced in: 5.5.2p2; 5.6.0b11; 2017.1.0a2
Not reproduced in: 5.4.5f1;

Note: video attached.