In some cases under the Application -> Trace view, the number of errors in the trace list and the error count in the detail view do not match.
When an EUM trace is correlated to (erroneous) backend traces, then the error count in the trace list and the error count in the detail view are different. The reason for this is, that the backend traces are considered individual traces. They are not part of the EUM trace. Only the UI integrates the correlated backend traces into the detail view of the EUM trace. The EUM trace itself has no errors, therefore the error count is 0. The backend trace has errors.
We can connect the EUM trace with the backend trace but not the other way around (we can't connect the backend trace with the EUM trace), and that's why the filter can't be used to display erroneous spans that originated from the EUM.