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.