Seeking advice on a dangerous problem with an elusive cause.

After running hot water in the master shower for a few minutes, a strong rotten egg odor is emitted from the AC vent. The smell is present in all vents serviced by this AC unit. The smell dissipates and does not recur even with continued hot water usage. This will happen again on another day, but is not always repeatable.

The gas line and systems have been checked for leaks recently and come out clean. But the problem wasn't occurring at the time of the gas line test.

The Furnace/AC system is a dual system. Both units are old but have been under home warranty and have had many parts replaced. The water heaters are new and have been checked for the source of the smell.

The water heaters are installed in parallel rather than series. Based on speculation by the latest plumber to look into this problem could the cause be something related to small off-venting of gas pressure caused by the spike/dip in gas pressure based on both water heaters firing? This would fit the symptoms as the source of the gas leak would be the furnace/AC unit but caused by the gas usage of the dual water heaters.

What would cause a small discharge of gas in the AC unit tied to hot water usage?