re: Ceramic tile over slab on grade being compressed. Any ideas?

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all 4 had very minimal thinset adhered to the back.

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I would bet the rest of the tiles have similar amounts of mortar on them. When some release it relieves some pressure on some others. All signs point to a substandard install.
Should be a minimum of 90% coverage on the backs of the tiles, but even with that coverage it won’t stop the tenting caused by the movement of your house.
You need perimeter joints ( can be left open and covered by the baseboards, and if you have a large continuous area with a lot of windows you will also need soft joints in the field tile. These are joints filled with a flexible caulking formulated to match your grout.
These are recommended every 12 ft or so, but without a lot of windows I usually go as much as 20ft or so.