"The tile guy says this is not a problem with how the shower was
constructed or tile was installed-- just says "it happens" with new
homes."

Ah, yeah.

Maybe "it happens" in their new homes. And maybe "it happens" with their construction and tiling methods. But obviously, it's unacceptable.

If you have a string of communications, gather them, and present them with the the opportunity to have the shower redone. Your shower has failed in the past, and it appears that it will insist on failing yet again with their repeated band-aid attemptd at repair. They may be :making repairs". But they are not addressing why the failures are happening to begin with.

If moisture has not escaped from the shower waterproofing, then that's a huge win in your favor, as your house may not have suffered damage from that slow trickle-type of leak.

Failures in the grout can be from movement below, or from the wrong materials used. Or even the right materials installed inappropriately.

A third-party analysis would be best. But that will cost you money. And it might be money you want to spend. Having the same guy do the same thing with the same failed result, versus having someone in to do it over.

A good outside tile mechanic will likely not want to do a repair on a shower that has repeatedly failed. Why would they put their name on someone else's failed work? A good mechanic will likely do an analysis of why the floor has repeatedly failed, then make a holistic repair.

On your side, you have certainly given your builder ample opportunity to make things right. They have failed to do so repeatedly. And it's important that you gave them the opportunity over and over.

My first choice would be for the builder to pay for a contractor of my choosing to make the repairs, whatever they may be. Like most jobs, you can get three estimates for repair and present them as a package.

If he balks, small claims court could be an avenue. Or arbitration, if that is available in your area.

Regardless, it won't be an easy process.