This condition in the stack is created by the discharge of waste from a fixture on an upper floor, causes a small part of the water in a fix¬≠ture trap on a lower floor to be discharged into the stack. Sufficient water remains in the lower fixture trap to fill the outer leg and the lowest portion of the trap when siphoning conditions exist subsequently, this is called negative pressure where the trap seal is pulled out of the traps being serviced
There is another condition called oscillation which is caused by a down draft (wind effect) as the vent terminal allows air to be pulled down this vent in draws out the trap seals of fixtures.
Even though the trap seal is reduced as long as it covers the trap crown weir you should not experience any foul odors.
This is a very common problem on high rise buildings and where a vent terminal is under 4" and where no provisions were allowed for a Fresh Air Inlet (FAI)
One of the easiest ways to rectify this phenomena is to make sure the vent terminal is clear and a four (4") VTR is properly installed or placing a return bend a 180 degree fitting on the vent terminal that will stop down drafts or debris going down this VT.
Josam item number 26700 like other manufacturers have what is known as a Vandal proof vent terminal cap that will prevent down drafts and will not promote hoarfrost, yet allow the free movement of foul air to be discharged
This is another reason I'm totally against any mechanical devices placed on a plumbing vent system like AAV's
I hope the installing plumbers did install a four (4") main vent undiminished in size through the roof and did not use wet venting and installed individual vents for each fixture.