Two of my toilets periodically have very low water levels in the toilet bowl.
I have read that it might be related to the vent... I do have a sewage smell, periodically in the room where the pipes go into the floor (is that the vent or is the vent a separate pipe. ) I think I found the crack... it is right by the floor... I put some flexible putty to temporarily plug the crack but haven't fully fixed it. I bought "mightly putty" that has an activator that says it bonds to almost any surface.
I also have had problems with the washer water coming up in the toilet but hope that has been fixed with a main sewer drain replacement... but still over the low water level in the toilet problem" Perhaps they are unrelated.