Guest
This may sound simplistic, but the whole reason systems are designed with vent stacks are so that a siphon situation isn’t created. I would initially be suspect of the specific vent for this toilet. Sometimes rodents/birds build nests or just fall into roof stacks and create a clog. If you can get on the roof with a flashlight you may be able to confirm. Otherwise, pull off toilet and run snake up stack from that piont- can be a messy job. Good luck.
