User updated the bill rate for an activity in the Activity Items screen, but the change did not reflect on the invoices created.
This happens because the rates on the invoices are drawn from the Time Entries screen. Updating the rate at the activity item level does not affect the existing time entries and, therefore, the invoices. The Time Entries screen itself draws the rates according to the rate hierarchy (Check out this article for more information on Rates in Core). For existing time entries, you need to use the Actions > Update Rates option to update the rates. Check out this article for more information on updating rates for time entries.