It appears you have some sort of leak.
Water leaking through tile grout.
Maybe a plumbing leak or maybe the drain is restricted in the shower and water is not allowed to fully drain away.
Leaky tiles can be found anywhere where there is water.
The shower water leak should be repaired quickly otherwise the buildup of stagnant water will lead to mold and damp under the tiles.
Cracks can develop in the grout between the tiles allowing moisture to seep through.
Leaking drainpipes in a shower or a bathroom plug will let water underneath tile.
The grout joints are porous so water can go through the grout and under the tile.
Can shower water leak through grout.
The best way to tell if.
This tile needs periodic maintenance.
There should be proper flashing the membrane should be sloped to a drain that has weep holes to allow the water through.
Check the ceramic tile walls surrounding your tub or shower.
What are the causes of water under a tile floor.
If it is leaking then it isn t fully waterproof.
They may have waterproofed the underlying deck but they could have made mistakes.