Like you I was confused about waterproof grout as I used to make shuttering for the swimming pool section and I rang my old mate up who used to do swimming pool tiling and he said grout are not 100% waterproof When I took the tiles of the shower walls in my property the plasterboards in the middle section was damp black patches and very black at the bottom because the water has crepted into the grout over the period of time, I also rang the manufacter and they said the same thing and now saying plasterboard are not suitable in the shower area unless properly sealed. If you tap the bottom of the tiles of the shower area, you may hear a difference echo sound as you do from the top as there's not much water enter at top.
No, I was more interested in the water being wet behind the tiles, I guess you mean the walls are wet behind the tiles, but I was being facetious as usual
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