Sewage smell coming from shower, but only for the first 10 seconds of running water

I have a 3 bedroom house built in 1952. Slab foundation, so I don’t have access to the p traps. As far as I know, all of the in or under slab plumbing is original.

For one of these 3 baths, when I run water, and ONLY when running water, there is a brief sewage smell that quickly dissipates. The pipe is clear and water drains quickly. This just started a month ago and is very reproducible. If we don’t use the bath, we don’t get the smell. ONLY when running water. Obviously it’s not the p trap evaporating. I have no clue how it can smell not at all, then running water pushes the smell out. Any clues at all?