In inspector/debugger/watch-expressions-preserve-expansion.html, we see that the V8 bindings reset the global object twice. This calls out to WebInspector.ScriptsPanel._debuggerReset, which resets the watch expressions, causing them to refresh their values. Since there's no context at that point, the evaluation in WebInspector.WatchExpressionsSection.update() fails. All of this is exactly as it should be, other than the twice bit... Ugh.
Chromium and V8 have left the building. Won't fix.