Weird to ask former employee for recommendation?

Might be easier to post the instructions you are responding to (each school has a different way of asking for recommendations) so people can see if your approach fits with the request before answering.
My initial thoughts: With the subordinate, past or present, there are still a number of questions this approach raises
-Why is this person a better recommender than another supervisor? How much insight into the applicant do they have, particularly compared to a supervisor (I can speak about the work of my subordinates with much greater detail and insight than I can into the work of my boss, who presumably doesn’t share all aspects of his role with me)? Your bosses can compare you with other people in your position (past and present), while your subordinates may not have that experience to draw on.
-Despite the fact that this person no longer works for you, the adcom can still imply a bias to his or her recommendation. Does this person owe them for help with his/her career and they are returning the favor?

You might also want to ask this in the admissions consultant section, as they likely have answered this for clients before. Good luck

That's a really great point. I think I will ask two previous supervisors to write my recommendation instead. Haha I probably should have gone with my gut instinct, which is that it IS weird to have a former subordinate write a recommendation.