You wrote: "The custom shower floor included a vinyl pan, mudpack, mastic...then grouted tiles. " Find out what specific adhesive was used to attach the tiles to the mudpack. There is "mastic" and there is "thinset". Some use the terms interchangeably, but they are very different animals. Mastic is a premixed product, it's thick and goopy and usually white colored. Mastic will come in a resealable plastic tub. Mastic is water soluble and pretty much every mastic maker specifies that it's not to be used on shower floors. If your floor gets wet, which would be expected in a shower floor, the moisture can soften or re-emulsify the mastic. That could be why the tiles are losing their bond and why tiles and grout are coming loose. Thinset should be used to bond tiles in a wet environment. Thinset comes in bags as a dry powder. You mix it with water. Thinset is portland cement based, so it cures hard like cement. Once cured, water has no effect on it. If mastic was used, that's dreadful. One other thing again based upon your description: Your vinyl membrane below the tile should have been installed on a sloped mud bed so the membrane itself is sloped to the drain. That's a building code requirement under the IRC. In general, the main components of a shower floor from bottom to top should should be: subfloor, then sloped mud, then the vinyl membrane over the sloped mud, then another layer of mud on top of the sloped membrane, then tile. If the membrane is flat on the subfloor, that can result in the membrane holding water and your floor staying moist. That moisture would exacerbate the problem if mastic was indeed used.

enduring, yes that's exactly what happened. The entire bathroom lacked any type of water proofing, whatsoever. They tiled directly to plywood subfloor in the bathroom area (the area under the toilet was also rotten once I uncovered it) and the concrete backerboard they had used was almost entirely disintegrated. Just for fun, I guess, they also did varying lengths of sheet rock around the shower. I ripped that all down as well, and will just be using cement backerboard all the way to the ceiling. Hopefully this will be a great project. Purchased new subfloor (3/4 tongue & groove), 1/2 inch wonder board, redgard, seam tape, and will be sizing in some (http://www.homedepot.com/p/MARAZZI-VitaElegante-Grigio-12-in-x-24-in-Porcelain-Floor-and-Wall-Tile-15-6-sq-ft-case-ULRT1224HD1PR/205473907) tile from HD. It was a 48 inch pan, but instead of replacing it, I'm going to do a tile floor replacement. Letting the tile from the bathroom floor extend once past a bumper and all the way up the walls. All of this from a peeling piece of old wallpaper!

you can clean it out well id use diluted bleach and scrape all the loose stuff off,vacuum /dry it out. go back to home creepshow and buy a small tub redgard and paint it in there with a chip brush and use mesh tape (as long as the backing is solid in there). slather it onto edge of tile and cover the flat surface & Let dry. Go to a tile store or online and buy a larger travertine or similar ceramic tan/brown soapdish big enough to cover it and thinset it in one fell swoop right over the hole and damage topically with a medium bet mortar also from creepshow its called prolite. Total cost maybe be $100 and youre elbow grease? im not a creepshow fan nor custom guy so much but its ez and 1 stop for you. otherewise tear the whole mess out ! this one is 8+ inches how bigs the hole?

Might be residual mortar or even broken-off portions of the ceramic bar? Looks like if you’re careful, you should be able to chip off enough of it with a sharp small chisel to set in your replacement tiles nice & flat.