Seal the toilet exhaust pipe. Consequences?

Hey everyone,
our plumber didn’t want to lay an exhaust pipe from the guest bathroom to the outside at the time, and it was installed behind the drywall with a special adapter. I was told that it is airtight because the air can only go in one direction.
Unfortunately, it still happens occasionally that the guest bathroom smells due to air pressure or something similar. I am therefore considering completely sealing the ventilation pipe, but what would happen then? Would it just gurgle when flushing, or should the ventilation pipe not be closed?
Thanks in advance.

That should not be exhaust air, but ventilation. You can do it with a valve attachment instead of venting it outside. But it is necessary, otherwise the water will not drain properly. If necessary, check the ventilation valve for proper function.

Thank you for the answer. Yes, it will be the ventilation. Of course, it is behind the drywall, but then I will install an inspection flap and check. Do I understand correctly that if the ventilation valve is working properly, it cannot smell unpleasant, or can it happen?