Fluid needs draining from tummy tuck 16 months later
without an examination, it is difficult to be sure, but chances are that you have what is called a bursae. It is a sac, that fluid keeps accumulating in. The fat layer did not stick down to the under layer. It will require a revision surgery, to remove the sac, so that the fat sticks down to the muscle fascial layer. I would discuss this with your surgeon, or get a second opinion where an examination can take place.
While I don't see anything in your photos, what you describe sound like a chronic seroma. It forms a lining which prevents it from sealing closed after aspiration so continues to recur unless/until that lining is surgically removed. Ask your surgeon to reassess you for that option and perhaps seek another opinion. I have never heard of a seroma resolving spontaneously after this long.