Dual-Wielding - Blocking bug

If I block with to swords I can only block with one, but if I press F5 I block with the two.

I thought I block with only one sword in F5, too.

In F5 I block with the two and not only with one.(you can see it in the attachment)

1. start a 1.9 snapshot(tested on 15w32a)
2. create or open a world(the best is creative)
3. hold a sword in the main and in the off hand(tested with diamond swords)
4. press F5
5. right click