The LINER is the waterproofing...as I said, neither the tile nor the grout is waterproof. Plumbing code REQUIRES the waterproofing to be sloped in a shower. While there may be inspectors that don't have a clue (if it doesn't leak, it's good), if they know and enforce their job, it would NOT pass an inspection. Some moisture WILL get beneath the tile. It will accumulate on the flat liner, and leave some there. Eventually, it will become smelly like a swamp. How long that takes depends on how often it's used, and how good the ventilation is.
Why is it that they water test the liner, but then not the tile? That's because the liner is the waterproofing.
There's decades of research on this. The Tile Council of North America is the industry bible...NONE of the methods (and they list many) has the liner flat on the floor, and it's for a reason...it doesn't work reliably. As I said, it may not leak into the room, but that's not all that is required to have a successful shower.