I ask the question on the title because there a quite a number of statements the Apostle Paul made about the coming of Jesus and resurrection of the holy ones that never fulfilled.
of course it has not fulfilled yet, because Jesus has not returned.
Evidently this "sacred secret" failed. They all died. And more than 2000 years ahead, here we are.
So on one hand you are claiming that something that Paul stated was not fulfilled but then on the other you are acknowledging that what Paul stated to be fulfilled is yet future.
So if your issue is not that it hasn't been fulfilled yet but rather that it had not been fulfilled in the time frame that you feel Paul had in mind...then what you have to keep in mind is that the 'sacred secret' that Paul was speaking about was about the resurrection of the holy ones occurring at the coming of Jesus at last trumpet.
The 'sacred secret' did not include knowledge of the timing of Jesus' coming. The mere fact that Paul spoke as if that was to occur in his time by the fact the he said 'we the living" is consistent with what was the mindset for ALL first century Christians and ALL since then down to our day (1,986 years later) and that is that no one knows the day and hour but are to keep in expectation/keep the watch, prove themselves ready, etc...has if Jesus' arrival is coming during your own lifetime.