If You Lose Volume After a Bbl, will Your Butt Be Uneven?

If you are even to begin with, and even after the surgery, then any volume that you subsequently loose will also be lost evenly.  Highly unlikely that there would be significant asymmetry.

the whole fat injected into your butts have genetical memory so if you stay lean or loose weight  after your BBL yes  they will reduce volume but not completely as you think if not no more than perhaps a 30 to 40% of original injected  volume

No.  When having the Brazilian Butt Lift, it is highly unlikely that you will lose more fat on one side than the other.  I have done close to 1000 Brazilian Butt Lifts and have never seen anyone come back with any significant volume difference from one side to the other.  No one is exactly even before surgery and no one is exactly even after surgery.  But I have never seen anyone who was noticeably uneven after surgery.

Following a Brazilian Butt Lift (autologous fat injection to the buttocks) one may experience a moderate amount of volume loss the first two to three weeks after the procedure as the post operative swelling subsides.  However, approximately three quarters of the fat cells injected should survive and remain over the long term.  Wearing a compression garment, buttock massage, and avoiding sitting on hard or irregular surfaces will help reduce the risk of uneven results.  Best wishes.