I don't know, but I can take a wild guess.
This is either a CS problem, an IT problem, or a financial transaction problem. It could be one or more.
If you schedule with the voucher and cancel or move outside the 24 hour window, all is good. It goes to TTF in the PAX name. We know that and live with it. Once used it is linked.
And, that is the key. I suspect (guess follows) that when the voucher is used to claim a reservation, the voucher buys a TTF that is immediately applied. I suspect that because when you cancel you get the TTF or what the IT system sees as the "original" payment method.
The voucher claim number may even disappear to be recycled in the system. After all, it is simply a CC# format with a 6-digit PIN that can be used over and over until they run out of PIN's. They may even have a fixed block of numbers assigned by VISA or MC or whatever the processing entity may be.
The solution would be to hold that account number for 24 hours so that charge backs could be made in the 24 hour window, but that is a lot of held numbers and it is ripe for crazy errors given that it is WN IT.
So, they clear the number and issue the TTF linked to the date of the voucher. If they were to issue a new voucher, they likely have no way to date it to the original voucher and the system defaults to today's date.
So what? We would quickly learn, this is FT, that instead of paying to extend a voucher we need only to buy a ticket, cancel within 24 hours, and get a new voucher with a year to go.
The simple fix is to date the new voucher the same as the old. It will match the TTF expiration date. Simple IT fix, except that it is WN IT.
At least that is my guess.