Membranes are not designed to hold water
Most above-ground systems are designed to shed water, not to be permanently submerged. Flat or reversed falls leave water sitting over laps, terminations and outlets — the three details most likely to fail.
Correcting falls
Where levels allow, remedial screeds re-establish fall to the outlet. Where they do not, thresholds, outlets or finished levels have to change. Burying a fall problem under tiles moves the cost, not the problem.