Yeah that was my first thought too, but ours isn’t tile and grout. I honestly wasn’t sure what it was at first, as I hadn’t seen another shower/tub like this despite all the houses we looked it, but to me it just looks like “cheap crap.” After poking around, I think it’s a “glue up” shower. It feels and looks like really thin overlapping sheets of vinyl/plastic/whatever. Which does feel like that’s more seams where a leak could originate, but despite that I still think it’s plumbing related and either at the handle or further up at the shower head. I’ll plan to tear into it again tonight after putting the kids to bed.

I’m going to be an idiot here…is the siphon the hardware on the front side of the tub above the drain and below the tub spout that prevents overflow?