New Construction-Sewer Gas-puzzle

I hope someone can give me a clue to a puzzling problem.
Bought a newly built home just over a month ago. No problems for the first couple of weeks then in week 3-4 after very heavy rain fall (very heavy rain fall is NOT unusual for my area -West coast of Florida) sewer gas creeps into my home through the front (closest to the septic) bathroom. The smell isn't immediate but creeps up over several hours until it is unbearable. We have had the plumber who installed the plumbing inspect /investigate twice to no avail. They snaked drains, used peppermint in the venting system to locate a problem.
This is not a constant problem but seems to only happen after very heavy rain.
We were not able to determine where the gas was coming from, when the plumber shows up hours later the problem has cleared up. (after airing out the house, one can't live in the stench)
This morning, after heavy rains last night, I had an occurrence of the smell and placed my face right next to each drain/faucet opening including the toilet bowl and around the bottom of the toilet. The smell seems to be coming in from around the bottom of the toilet. I called the plumber's emergency line and spoke to the owner regarding WHERE I smell the gas coming from. She informed me that the builder is having similar problems with other home of his in the area. She also had me open the "cleaning cap" (I think that is what it is called. where they clean out your tank?) to vent and that seemed to , almost immediately, elevate the problem.
Has anyone ever heard of such a problem? What is the cause of this? Am I wrong in assuming that no matter the climatic conditions a septic/plumbing system should never allow sewer gas into the home unless the entire system is in fact failing?
Help- I've never had a septic system and I am trying to learn all that I can!
Thanks !!
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