There are no facing rules in 5e by default. Thus one provokes an OA leaving an opponent's reach no matter how you imagine the characters are faced. The reasoning behind this is that the round represents six seconds of movement: thrusts, parrying, bobbing-and-weaving, &c.*
In your example, the answer is: yes, Atone would provoke an OA from Dip.
If you really want facing turn to the DMG. P.252 describes "Optional Rule: Facing." However, this optional rule says nothing about OAs. In my opinion it wouldn't be much of a stretch to discuss a reasonable modification to include OAs if your table chooses to use these optional rules.
*-This is explicitly stated at the beginning of "Movement and Position" (PHB p.190): "In combat, characters and monsters are in constant motion...." It's also the natural inference drawn from the description of "Other Activity on Your Turn," the existence of reactions, the inclusion of DEX mod into some ACs, and the first paragraph of "Opportunity Attacks" (PHB p.195).
From the section on opportunity attacks:
You can make an opportunity attack when a hostile creature that you
can see moves out of your reach. To make the opportunity attack, you
use your reaction to make one melee attack against the provoking
creature. The attack interrupts the provoking creature’s movement,
occurring right before the creature leaves your reach.
"Creature moves", "provoking creature", and "interrupts the provoking creature's movement", all support the position that the intent of opportunity attacks is to react to another creature's movement, not your own. This is further clarified in the subsequent paragraph to mean "voluntary movement".
If the intent of Polearm Master was to subvert the general rule with a specific one, it would be clearly stated, as in "This is an exception to the general rule on opportunity attacks, in that it does not require movement on the part of the target". The phrases "enters your reach" and "moves into your reach" are--barring any explicit wording to the contrary, pretty clearly synonymous. A relative movement interpretation does not match anything else in the book, unless it is clearly described as such.
In short, "Enter" is an active verb, and had Polearm Mastery" been intended to break of the general rule, it would certainly have been pointed out by the designers in errata, interviews, tweets or Sage Advice.
In the absence of anything like that, the general rule interpretation should apply.
To solidly support this, user Korvin Starmast has kindly supplied definitive clarification:
Sage Advice Compendium, page 8:
Does Polearm Master let me make an opportunity attack against a target that is being forced to approach me? A creature doesn’t
provoke an opportunity attack if it is moved without the use of its
movement, its action, or its reaction"
Your enemy is being forced to move by you and he is not using his own movement, action or reaction, therefore he won't trigger an Opportunity Attack.