If the pressure in the system becomes negative (vacuum), any opening to the system will suck air. If the air elimination device has a cap and it is left open, and the pressure goes negative, it will suck air.
Fix the reason that the system goes negative and you will solve the air problem. Keep the caps tight.
Remember, the lower the pressure, the lower the boiling point.
pumping towards an expansion tank can cause low or negative pressure at an air vent location, seen in this drawing from "Modern Hydronic Heating" With a high head circ capable of enough delta P you can pull a negative pressure. both air and cavitation problems may occur.
Typically a float type air vent wants a +5 psi pressure to assure a tight shut off.
You can buy anti-siphon caps as a "fixer' or a means to confirm that air is being allowed in at the vent.
A hydroscopic cap is another nice addition for a second level of protection at the vent. If the needle valve shut off fails or gets dirt under it, the hydroscopic cal will serve as a second shut off. And they can be combined, the anti siphon (check valve) cap, and the hydroscopic.
Let me know if you need one to try, I believe the mip threads are common on vents?