When you select all vouchers in the list (first checkbox in the title row). It obviously selects not only the 10 shown vochers, it selects ALL vochers you have. Clicking the "delete" Button than REMOVES ALL YOUR VOUCHERS! Not just the currently visible ones.
This behavior is absolutely wrong and needs to be fixed asap!
See: Business Wi-Fi -> BUG in Voucher Manager: Unintentional deletion of all vouchers! for ideas how to fix it.