The biggest issue with a walk out shower, is the door getting wet and dripping all over the floor at the tub and tile seam.
Looks like the plumber did not take a shower and get water all over the place inside...or they would have found the issue.
This is my guess, but it could also be a combination of the two, anytime I test a pan, I block the drain and let the water fill up to the top of the pan, just like in real life when folks are standing on the drain.
Let us know what you find.