Assert LinkedHashMap element in jUnit test
One cannot say anything against

assertEquals(failedEvents.size(), 1);
assertEquals(failedEvents.get(0).get("eventId"), "EVENT3");


It is even shorter and more clear.

However your argument of an invariant on the entire collection is valid, though
you should have said so. Better would be to make a reusable assertion:

assertEquals(failedEvents.size(), 1);
assertTrue(failedEventsHaveId(failedEvents, "EVENT3"));

/**
 * Invariant on the failedEvent: the eventId remaining one.
 */
private static boolean failedEventsHaveId(..., String eventId) {
    return ...;
}


If not reusable, then the code is unnecessarily "nice."