You should create a codpen showing the issue; it would be important to see how you are defining and instantiating the class. That being said, JS Classes are not like classes in class-based languages; it sounds like you are modifying a property of the prototype.
Turns out I am an idiot and didnt read documentation on things.
I was initializing my velocity to Phaser.Math.Vector2.ZERO, which i shouldnt have been doing as it mean all the objects used the same velocity reference. (the documentatioin for that specifically says its a static reference and should only be used for comparison.
What I thought was a weird scoping issue with javascript/typescript was merely a bad reference on my part.