This is not a donor-facing issue. This is merely an in-house display discrepancy.

The message preview feature before re-sending a receipt is displaying recurring donations as one-time, not recurring.

For example, donor cid=736672 requested a receipt. Upon resending it, Seilo noticed that the donor's new recurring donation in the Message Preview feature lists the donation as 'one-time' in the opening paragraph of the receipt.

All is well on the donor-facing front of this issue. When viewing the re-sent receipt in Activities, the receipt the donor received correctly states '... to support Wikipedia every month'.