Cracked grout and tile movement (8 year old installation now leaking through)

Quick answer is poor installation. The leak is probably poor pipework, if only one of the two are having that issue. There is a high probability that water is getting between the tiles as well but assuming pipework issues are the main cause (if not, then why is the other bathroom not showing the same issue?). Your floors are incorrectly installed. If you have movement of the floor then the tiles will crack or come loose. Your grout has done it's job by cracking, that is what it's meant to do if movement is happening. I'll assume the floor is wood rather than concrete. So they have either tiled direct to the floorboards or installed some plywood and tiled onto. Both are incorrect. Plywood is no longer good enough quality for tiling onto. There should be some cement board installed and then tiled onto. I would also hazard a guess that your moving tiles have been stuck down using "dot & dab"... which is wrong and creates movement. Your wall tiles are probably installed in a similar fashion, dot and dabbed onto a sheet of plasterboard. No tanking (waterproofing) system either. Again, movement of tiles that are not fully supported on the walls, causing grout to crack and water to penetrate. Regrouting is a waste of time as it will perish very quickly. The sad truth about this is that the installers have not done anything correctly. If the tiles are wrong I would beg the plumbing is sketchy too. Sounds like you need to re-do your bathroom(s) I'm afraid. You should be able to get a good 20 years out of a well installed bathroom (a regrout would be required in that time frame but that would be to freshen things up).

If the grout has cracked there is movement a simple regrout won’t work. You will also need to find out where the water is getting to the floor so may need to be re sealed first to stop the water getting onto the floor.

If there's movement in the floor itself then that's going to cause the grout to crack and also weaken the bond between the tiles and adhesive, my advise is to rip up the floor and redo as regrouting won't stop the problem persisting. As for the shower enclosure again if the grouts faild there's a very high chance water has worked its way through behind the tiles hence you now have movement in the tiles, probably best to rip out and start over as the wall will need attention before being able to re tile, if the jobs done properly it should last double the amount of time if not longer. Hope this helps

Could be water ingress due to the falling grout I don't believe is the instalation as you will have water leaking all the tape Probably need regrouting and sealant to cover entirely

Installation issue I would say. Probably not used flexible adhesive or grout on floor