Sewer Gas From Shower Drain - Haunted?

US based. House is 20yr old, lived here for a year.

Every time the SECOND person showers in the en suite bathroom, sewer gas comes up the drain. Every. Single. Time.

It only happens in the shower in the en suite bathroom. Only to the second person showering that day, about halfway through the shower.

Doesn't smell at any other time**, no other fixtures, no other bathrooms.** No water backs up.

I have tried using enzymatic drain cleaners, sending down copious amounts of hot water, baking soda, etc etc etc.

Still does it. I've had a few plumbers tell me "it must be the p trap is missing," but then we'd smell it all the time, right? We don't. Only shower #2 of the day. (No pun intended, but I'm leaving it)

At this point I suspect it's the vent stack that only services this side of the house. When you flush the en suite toilet across the room, you sometimes hear a thud in the wall when the water is going down.

Is this a vent stack issue or is my drain haunted? (the last owner died here, so I'm thinking about 2% chance)