Why does contribution verification display a numeral value rather than a text value
While it's still clear you've got a bug, I'm pretty sure your issue isn't that you have "1" but rather that the unit--week, month, year--is missing.  The numeral is there in case you wanted to donate quarterly--it would say "3 month(s)". Again, not that this makes it any better.

However, it might help you or whoever diagnose the bug.  Hopefully it's as simple as the payment processor extension not providing the unit back to the receipt/thank-you page.