User has a project with the contract type Hourly Not to Exceed, but on creating non-extra time entries after having reached the contract amount, Core does not automatically mark them as non-billable and they keep on adding to the invoices.
This can happen if the project has the rule Skip automatic time entry evaluation active. This rule causes Core to stop the evaluation of time entries against the contract amount. You should remove this rule to trigger the evaluation and mark the entries as non-billable. If you want to further impose a strict limit on the contract amount, you can add the project rule Prevent time and expense entry after reaching contract amount. Check this article for more details: Net Bill is higher than contract amount for fixed projects.