I suspect the worker is reporting an error and it's being propagated to the window's onerror handler, sometimes before onload and sometimes after.
{
"status": "ERROR",
"group": "default",
"message": "unexpected token: ':'",
"stack": "http://web-platform.test:8000/workers/constructors/Worker/does-not-exist.js:1:8",
"subtest": null,
"test": "/workers/constructors/Worker/Worker-constructor.html",
"line": 142571,
"action": "test_result",
"expected": "OK"
}