dnd 5th – The siphon of blood Stirge also allows him to flank?

I play a Beast Master ranger with a restless animal companion.

I know that once an emotion is attached to an enemy, it does not need to use its action or anything; Damage caused by blood loss is passive / automatic, no action is required.

If it attaches to the back of an enemy, does it count as a flan (according to the optional DMG rule) when it is attached?