As an installer I can tell you we know when a piece is already separating while installing. Drag a piece of rebar or a hammer across the top of each stone. If it’s hollow, don’t install it.
If you’re the homeowner, build this into your contract. Also make sure it’s in a full bed of mortar. No hollow sounds. Establish this as an expectation when getting quotes. They’ll then tell their labor. Explain the horror stories. While no one warranties stone, you have a right to state that you expect them to catch material defects prior to installation and defects in workmanship [air gaps preventing the stone from fully bedding in the mortar].
You may pay an extra few dollars a sf, but it’s been my experience that most homeowners looking for a Thermal cut flagstone patio won’t haggle over a few extra dollars per sf.
I’ll stop short of requiring them to back butter every stone, but will require that they test each stone for no hollow sound as I drag a 1-2’ metal bar (or hammer, chisel, whatever).
New jobs, make sure to use a sealer. Period...the end. ;)
Q