When you use En Avant or any other movement-based ability, the game calculates where you'll end up and tries to send you to that exact position. If you're lagging, you'll end up covering less distance due to already covering some of the distance. The more lag you're going through, the more distorted your trajectory becomes as the game tries to send you where it thinks you should be.

It's not necessarily a bug, but it's definitely not working as intended.