Attaching hollow tiles / experience

In our shower area, two tiles are partially hollow. The tiler wants to renew them, but in the process the sealing membrane will be destroyed. He plans to coat it with Seccoral. I am not convinced by this, as I was really meticulous about the proper sealing and now, of course, I do not want any leaks. Has anyone ever used webersys 891 for hollow wall tiles and can share their experience with me?
By the way, these are wall tiles.

For wall tiles, just leave it as is, or is the grout already crumbling out? If I tap all our tiles, I will certainly find a few hollow ones and that's why I don't do it.

Hello. So if they are already hollow anyway, they should be removable without much effort.. The sealing layer can be renewed quite well if it is destroyed after all. It’s not a swimming pool but just a shower. Literally liters of water do not run through the joint.. But how can 2 tiles be hollow on the wall?

But how can 2 tiles be hollow on the wall?

Just like the ones on the floor.

In our rented apartment from 1998, we had the situation in 2000 that above the bathtub, 3…4…5 rows of tiles over about 2.5 m length completely detached from the wall. The whole structure was only held together by the grout. The "bulge" was definitely 20-30 cm big.

To the actual topic. New building and under warranty? Have it repaired.

If you want to fix it yourself, carefully remove the grout, then the tiles will "come towards you," otherwise leave it as is.

The tiles are still firmly attached in some spots, and if I remove them, the waterproofing will be damaged. If I leave them in the shower area, they could possibly break if bumped.

The tiles are still firmly attached in spots, and if I remove them the waterproofing will be damaged. If I leave them in the shower area, they might possibly break if bumped.

Who exactly is saying that? If they are loose, they are loose and will come off the wall easily. Nothing will be damaged. Tiles break if you hit them with a hammer, otherwise nothing breaks there!