Ours did exactly the same - cracked down the center, in-line with the burner. And that's the issue... the burner is linear and causes thermal stress that the ceramic stone just isn't built to handle. We still use ours - just now have to put it in in two pieces. Even though we butt the pieces together along the crack, it has not deteriorated further.
Some folks use smaller, square unglazed tiles you can get at home improvement stores and avoid this (but then they still have multiple pieces to handle so... six of one, half dozen of the other).
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam