When I move away from an enemy I am flanking and trigger an opportunity attack, do I have advantage on my attack from the Riposte maneuver?

Question

I am playing a Rogue who is planning on dipping 1-5 levels into fighter. If I get to level 3 of Fighter, I will take the Battle Master archetype and take the Parry, Riposte and Feinting attack maneuvers.

Our group uses the "Optional Rule: Flanking" from the DMG.

Say it is my turn and I am currently flanking an enemy with a friendly PC. When I move directly away from the enemy, they may attempt to hit me with an opportunity attack. If it hits, I use Parry and reduce the damage. If it misses, I use my reaction to Riposte to hit him back.

Would I have advantage on the attack from Riposte because I am still flanking? Or because I started moving away, would I no longer be flanking?

My party uses the optional rules of flanking and playing on a grid. (We're still working on getting custom miniatures, so we are using extra coloured D6s to represent enemies and PCs.)

Side note: I started thinking about this because I wanted to maximise my Sneak Attack damage. I can do Sneak Attack when I get hit outside of my turn and use Riposte. So I thought maybe I could trigger an attack so I could cause double Sneak Attack damage reliably. But then I realised it would still be on my turn, and the description for Sneak Attack is quite specific when it says "once per turn". I continued wondering about the flanking/advantage bit, though.

Answer 1

Yes

You flank when you are adjacent to an enemy:

Flanking on Squares. When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy's space, they flank that enemy

You are adjacent when you are within reach. You are within reach in the moment of the OA:

The attack interrupts the provoking creature's movement, occurring right before the creature leaves your reach.

So yes, if the enemy has 5ft reach, you are still flanking in the moment of the opportunity attack from this enemy.

Unless your DM says otherwise

The Standard Caveat: D&D 5th edition empowers the DM in ways that 3rd, 3.5, and 4th did not. While rule zero has always applied, 5th edition chooses not to explicitly codify many things. If your DM says you don't, you don't.