The timing for load and DOMContentLoaded events as reported in HAR and to extensions are reported as difference between main resource start time and the time corresponding page event has been fired. During reload of main resource, loadEventTime and domContentEventTime are not reset, so timing uses event times from previous page load and main resource start time from new load -- this results in negative values.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.