User made some time entries on a project and the employee cost rate is set to a non-zero value, but the project cost still displays zero value ($0) in Projects > Performance.
This can happen if you are using a fee schedule on the project and have set Cost Rate = $0 there for the Employee-Activity Item combination. The Settings rule Allow zero (0) rates in service fee schedules must also be checked. The cost rate of the time entries is set to zero as a result of this rule, leading to a zero total cost on the project. You should un-check this rule in Settings > Billing & Invoices > General so that if a zero cost rate is set in the fee schedule, the rate is fetched from Employees > General > Rates & Options instead.
Note: If the project rule Use bill rates from Activity Items screen is also set, that rate is next in hierarchy to the fee schedule rate, provided it is non-zero. Check out this article for details on rate hierarchy in CORE.