Joist ends are a common hidden weakness
The ends of joists often sit in or close to masonry. If the wall is persistently damp, decay can begin at the bearing while the middle of the joist remains sound.
Poor ventilation slows drying
Blocked air bricks, raised paving, insulation or later alterations can reduce air movement beneath the floor. Moisture from the ground or walls then remains around the timber for longer.
A soft floor may have more than one cause
Loose boards, undersized joists and damaged supports can all make a floor feel springy. Opening-up is often needed before the cause can be separated from true timber decay.
Repair the damp source before closing the floor
A new joist inserted into the same wet wall pocket can begin decaying again. The wall, ventilation and any plumbing or rainwater defect need to be dealt with as part of the repair.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.