My advice, it was just done wrong. Remove any vents in the floor or underneath at the soffit area., they should not be there. This is not a roof assembly and should not be treated like one.
You need to seal off the underside with plywood and a weather barrier like tyvek or tar paper to help keep the air out, or at least slow it down.
Then you can install fiberglass insulation with a paper faced vapor barrier toward the heated space. If you are looking down at the floor joists you should see the paper on top.
Good luck, and ask if you have more questions.