Interesting read, I’ve seen several done the wrong way. Most notably a slab at an ag dealer, they wanted a heated outdoor slab for power washing equipment. I don’t recall any gravel being used under it for starters, the vapor barrier went under it, then they were placing sand on the vapor barrier to fill any imperfections before placing the insulation, then to top it all off they filled any irregularities in the foam board with sand.
I’ve also read numerous other articles, vapor barrier on top on the insulation then they just stapled the pex down right thru the vapor barrier.
Still weird that it worked the first year then failed the second. Only thing I wonder is did you have a dry year the first year, then it got wet the second and water found its way under the slab?