## By default there is no facing in 5e

The standard rules do not use the concept of facing. All combatants are supposed to see in all directions, as they are moving in their space to respond to threats. It is not defined in which direction the front or back of a combatant is facing. 

This can be seen as the DMG on p.252 offers:

> **OPTIONAL RULE: FACING**
> If you want the precision of knowing which way a creature is facing, consider using this optional rule.

So, while the beholder has a front side (from which the anti-magic cone issues) and a back side, in combat by default it is not defined which way the front side faces. That is the reason why the monster's description instructs you to decide, each round which way the cone (and with it the eye) faces.

There still is mechanical advantage for attacking the beholder from "behind", i.e. the side in which he is not facing with his main eye that round. You can justify that due to the other 10 eyes the beholder has, which allow him to look in all directions.