Hi CB: I’ve always installed a shower membrane to the shower framing BEFORE any morter is poured. I’ve never seen the shower liner installed after the morter’s been poured. Also make sure you use a proper showerliner not vapour barrier materials. Once the morter has been poured ON TOP of the liner and has hardened, then the tiles are installed on top of the morter. For your info, I’ve been a plumber for over 25 years and have never seen it done the way you’ve described it. Hope this helps. Marty