How does the developer know if a failing case is due to a bug or something that needs to be changed? Is the new developer being dropped in with no support and n

How does the developer know if a failing case is due to a bug or something that needs to be changed? Is the new developer being dropped in with no support and no peer review and asked to make a non-trivial change? If so, then tests are not so useful in an strongly typed environment since interface changes will be caught at compile time. It is not obviously self evident to me.