I’ve just received my email notifying me of my January voucher and believe the process is as follows:
Vouchers are issued by email just after the end of every quarter (usually 2nd or 3rd) based on points accumulated up to a month before. If you have ticked the relevant box in preferences, they are also sent at that date by post.
The “Sunday Papers” email sometimes, but not always, includes a reminder at the bottom of any vouchers you have outstanding. It is usually only a repeat of a voucher previously sent by separate email, but it appears this Sunday’s email jumped the gun somewhat for those with no other outstanding vouchers, and included the details of the about to be issued one too.
Hope that helps.