A Beholder has no 'behind itself'.
The Beholder creates the anti-magic field with its central eye, so if you consider the location of the central eye 'front' and the other side 'back', then no, it can only ever project its anti-magic field to its front side.
But for all purposes, a Beholder doesn't really have a front or a back, because it's not limited to only being able to see in one direction. You and I have (presumably) two eyes, set in the front of our head, which creates a vulnerable back side, which is where our conception of a 'in front' and 'behind' comes from.
A Beholder on the other hand has lots of eyes that can all move independently of one-another. It has 360 vision and is not limited to a front or a back, it can see and disintegrate you even if you're below, above or 'behind' him.
In fact, this has to be how Beholders operate, because its own anti-magic cone will also cancel its own eyerays, so a Beholder can't even keep its intended ray-targets in its 'front'.
It has a side where food goes in and a lot of sides where deadly rays come out.