The bug
Experience orbs in front of other expereince orbs block the rendering of the underlying orbs.
Note that the translucency of experience orbs is an intentional feature and should be kept - see MC-163034.
Another Note: As of a release, this now only happens sometimes, so be sure to summon more than 2.
This seems to be fixed in 1.15 pre 3.
Edit: After some more testing I found out this was only partially true. Sometimes experience orbs do render behind other experience orbs, and other times they don't.