It appears you have some sort of leak.
Water leaking through tile grout.
If the grout becomes damaged on the floor it will give water a chance to slip through the gaps and under.
There should be proper flashing the membrane should be sloped to a drain that has weep holes to allow the water through.
Leaky tiles can be found anywhere where there is water.
Can shower water leak through grout.
Check the ceramic tile walls surrounding your tub or shower.
They may have waterproofed the underlying deck but they could have made mistakes.
The best way to tell if.
The shower water leak should be repaired quickly otherwise the buildup of stagnant water will lead to mold and damp under the tiles.
What are the causes of water under a tile floor.
This tile needs periodic maintenance.
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.
If it is leaking then it isn t fully waterproof.
Leakages in the shower are most often the result of grout failure.