If they have not written saying that they resign, how can the union reasonably be expected to know this?
I would say that a reasonable person would accept that they are still considered a member in the union's eyes, until the non payment is spotted and communications establish the underlying intent.
Direct debits may be unpaid for a number of different reasons.
As to the revised final offer, I imagine that the person accepting it had to sign a short declaration that they were not a union member on the date it was issued.