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