I saw a 12 year old bathroom with polished porcelain dot and dabbed straight onto floorboards riddled with woodworm, not a crack in sight apart from one tile put that was sinking into the void as the floorboard was dust.
That picture looks like it was done on ply .the ply has shrunk in size .the glue is good so the weak point is the tile and it cracks in a straight line .so close to edges of tile .it's definitely weak tiles and no decoupling membrane