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 Global 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 Global Settings > Billing 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 more details on the rate hierarchy in Core: Rates in Core.