I tested with the default texture pack and got the same behavior. I have no mod installed.
Although the XP orbs are flat, you can always see them from above in first person view or "third-person rear view" (e.g. 13.png above). The problem I'm reporting is that the "front view" looks bogus (the other two pictures). In other words, the XP orbs cannot be seen as "flat" from any other view than the "front view".
I wasn't complaining about the orbs not being absorbed: they are absorbed, but it is slow because there are too many of them here (I killed 200+ zombies at once with a splash potion).
13.png is a back view from above (orbs are appear round)
19.png is a front view from 45deg (orbs are not round).
Yes, I have one block at (80,9,416) which is under direct sunlight (or moonlight). The middle position in my sequence, in which the full room is visible, is just 5 blocks away from that block.
So I guess I can't really call this a bug, since it is documented. It is still a bit counter-intuitive though.