The liners are apparently being hardened with steam or hot water. That still shouldn't cause backflow, "rising air" isn't worse than diffusion and the little bi

The liners are apparently being hardened with steam or hot water. That still shouldn't cause backflow, "rising air" isn't worse than diffusion and the little bit of extra pressure from heat should head down-pipe before pushing through traps. But it does sound like a suspect for why this is showing up now in plumbing that hasn't had issues with other smells.