Your bill is tied to your account number. Some types of account activity (such as those shown below) can cause your account number to change. When your account number changes, it can seem like something is wrong in My CenturyLink, when actually, you're just being shown the old account number.
You know a new bill has been issued, but the site is only showing an old bill? That's a good indicator that the account has been updated.
To be sure, compare the account number you're seeing in My CenturyLink to your current account number as shown on the missing bill. Make sure to look at the entire account number, as sometimes only a few digits may change.