squeaking shower base

Hi everyone, Our plumber installed the acrylic shower base without a mortar bed and I noticed it moves up and down when pushing on the back corner (1st video) . Asked them to fix it but they pushed ahead with putting hardieboard on the walls and floors. They added some spray foam on the back...

Hi all. Here is my dilemma. Installed a 60x36 acrylic shower pan. It has a 3” front lip and 3 feet at the back. Plywood reinforced bottom with a center drain. Used shims and sand mix for leveling. I did not pour a whole bed but rather used piles of sandmix to level the unit. I tested it prior...