Recurrence usually means the conditions returned
Fungal decay needs suitable moisture. If a gutter starts leaking again, a plumbing defect is missed or an underfloor void remains poorly ventilated, timber can become vulnerable once more.
A repaired area may not be the only damp area
The first outbreak can draw attention to one room while another source of moisture remains concealed nearby. A thorough survey should therefore consider the building around the outbreak, not only the timber that was removed.
New timber still needs a dry environment
Replacement joists and boards are not protected by being new. If they are built into damp masonry or enclosed before the structure has dried, the same conditions can be recreated.
Maintenance matters after treatment
Keeping roofs, gutters, downpipes, plumbing and ventilation in working order is the practical defence against recurrence. Periodic checking is more useful than assuming the problem can never return.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.