A cracked tile is almost always a moving substrate
Tile is rigid and grout is more rigid still. Neither tolerates the substrate beneath it moving, and a wood-framed Toronto house moves seasonally whatever you do. The Ontario Building Code designs floor framing to L/360 deflection under live load, which is adequate for most finishes but marginal for tile over a long span. That is what a decoupling membrane is for: it lets the substrate move a fraction without dragging the tile bed with it. Skipping it saves a few hundred dollars and is the single most common reason a tile floor in this city develops a crack line two winters in.

