User has applied write-up write-down (WUD) on time entries by updating their amount in the Time Entries screen. However, when the user creates an invoice from those entries and later reverses it, the WUD on the time entries is lost and they show a 0% WUD.
This can happen if you apply a WUD to the time entries while creating the invoice (by updating Net Bill or % Complete in the Create Batch Invoice screen), and when the Global Settings > Billing rule Reverse write-up/write-down when invoice is reversed is checked. Normally, this rule reverses any WUD you apply to the entries in the Create Batch Invoice screen, but does not reverse the WUD if you only apply it in the Time Entries screen. However, when you apply the WUD at both levels and the Global Settings rule is checked, Core completely removes it for the entries at both levels. If you want to prevent this, you should either un-check this rule in Global Settings, or avoid applying the WUD at both levels. Check the Core Help for details on the Global Settings rule.