I thought you were in MA, not ME. The good news is that if it's the PT that shrank, it's done. The bad news is that if the soil is undergoing expansions and contraction cycles...it's unstable. I don't have a vast amount of experience working in clay, but I had a big bucks job in clay years ago. We actually set up something similar to a drip irrigation system around the perimeter of the foundation. The purpose was that the drip irrigation would keep the clay continually moist so it wouldn't undergo movement cycles as the clay dried and rewet. It'd essentially stay permanently wet and not move. Any excess moisture was removed by foundation perimeter and footing drains. Now, you wrote that the crawl space is sealed up... 1) For clarification, I'm assuming that there is a concrete foundation around the perimeter of this addition? Or is it built on piers? 2) If it's a poured foundation, was a rat slab (thin 2" poured slab) or a full slab poured in the crawl space? Or is the bottom of the subfloor open to soil on the ground? 3) With the clay, did your dad have any engineering analysis done with regards to soil movement or stabilization? As to "what now"...was this a Kerdi shower? Or did it have a topical membrane (HydroBan, RedGard, etc)? I remember your shower but forget the specifics of how it was built. If Kerdi, the cracks are sort of inconsequential when it comes to water penetrating into the walls. The cracks are more cosmetic unless they really open up. I'd get in touch with your installer regarding the details of the walls. Depending on how the walls were detailed/waterproofed might give you a warm fuzzy regarding letting things sit as they are while you see if movement has stabilized versus taking action (caulk the crack, etc) if water penetration may be a concern. I'd mark the ends of the cracks with blue tape and keep an eye to see if the cracks extend or if the movement has stopped or stabilized.

Pics?

Unsanded grout should only be used in grout joints less than 1/8th inch wide. Unsanded grout does not have bulk filler in it. So if it is packed into too-wide joints, as the grout cures and water leaves the mix, you can get shrinkage cracks and flaking grout. If too much water was used in the grout mix (sanded or unsanded), or if too much water was used during grout clean up, or if the grout was not thoroughly packed into the grout joints during installation, then the grout may crack, flake, or show pinholes during the curing process. Solution? If the grout was properly installed but there are just a few pinholes or voids, then additional grout can be applied over the existing, but that's best done within a day or two of the original installation. If the existing grout is structurally weak; cracking, flaking, etc...it's best to scrape the old grout out to a depth deeper than the grout joint is wide, and install new grout.

If you have a curb, you will have the cracks develop there, too. and where the walls meet the floor. Anywhere where two different materials meet or where one surface meets another that is in a different direction (known as "change of plane"). My contractor still grouts such places, too. But I did a solid surface shower that he could not do wrong. I am keeping an eye on the place where the floor meets the shower pan, all other places are in dry areas. So there it is just cosmetic. I would guess that water can get behind your tile with those cracks. If properly waterproofed it is fine, but if not, I would get to this as soon as you can. A lot of shower builders do not use waterproofing, even though it is now required. If you do a search on this forum for "Shower waterproof," you can see many discussions about it and learn what is and is not waterproof. (Redguard yes, Durock no, for instance.)