Stubby wrote:

After about a year it appears that the PT lumber was expanding and
cracking the tiles and grout. One of these days I'm going to have to
redo the whole enclosure. But first, I'd like to understand what
happened and what the "right" material to use is.

the pressure treated lumber is wet so it shrank at first
this caused cracks
once the craks were there, water did get in and
the pressure treated lumber swelled up
probably cracking the grout more.

next time, regular lumber will be fine for subframe

__ this may have posted already, i hit the stop send button

i wanted to add... if you can keep the area dry for a couple of
months...the treated lumber will settle, it will dry and pull all
it's going to pull...at that point you could just re-grout
rather than have to tear the whole thing out