So check this out, a friend of mine that is a great plumber had a odd thing happen. First of all all waste and vents are installed and sized correctly. They were also scoped by a video camera, PERFECT waste and vent.
Here is the thing.....back to back toilets ( ABS Waste) Kohler "Hi-line" Water Closets installed.
One or the other of the Toilets will loose its trap seal (it gets sucked or blown out) Kohler says "yep this happens sometimes with the 1.6 gpf large Flapper type toilets ......WHAT !
I suspect this is possible but more likely a piece of string or dental floss is siphoning out thew trap.
If it is just the quick or fast flush toilet then the Toto " Drake " would also do the same thing.