XP Orbs have graphical bugs when looking straight down

When looking down with an angle of exactly 90 degrees, xp orbs become invisible and/or start flickering when you are a few blocks above the ground. They don't disappear when crouching, but they still flicker. This bug happens in both fast and fancy, and their shadows stay visible in fancy.

Steps to reproduce:
1. Throw a bottle o' enchanting on the ground
2. Fly about 4 blocks above the xp orbs
3. Look straight down. When moving around, the xp orbs flicker, and when standing still, they disappear. Note that you have to look down with an angle of EXACTLY 90.0 degrees, you can check this in the F3 menu at "Facing:".