When you have a bow in your main(right) hand and a shield in the other hand, you draw back the bow to shoot, the shield disappears.
The shield is visible before you draw back the bow and it reappears when the bow gets back in normal position.
The shield is invisible only in the time period when the bow in stretching.
This wasn't an issue before 15w49, in the previous snapshot the shield is visible in the said time period.