Most of the codes I have read state the bathroom flooring must be non absorbent materials for this obvious reason.
If you have water lines especially cold water passing under this floor during humid days the condensation could cause enough moisture to cause this type of condition.
Very common in high rise buildings with a central absorption cooling unit.
Check your piping carefully and feel around for dripping lines this is an indication some fixture maybe running like a defective flapper or diaphragm inside a toilet tank or flushometer or a piston assembly on a Sloan Gem or other fixture may have a slight drip
What you should be looking for is condensation…