I guess the air results from the fact that a minor portion of your liquid sticks to the inside of the tip. Consequently, this liquid is replaced by air during dispense.
Is it possible in your case to use independent aspiration and dispensation (surface mode) for mixing instead of using the mixing as part of the aspiration single step?
Then you could aspirate a little more volume than dispensing to perform the mixing procedure.
For example
Aspirate 520 µl
Disense 500 µl
Aspirate 500 µl
Disense 500 µl
Aspirate desired volume
Dispense desired volume