On Enterprise Ubuntu 18.04 PPC64LE, the TestOplogTruncation unittest in the WiredTigerKVEngineTest suite occasionally times out, here:

    callbackShouldFail.store(false);
    assertPinnedMovesSoon(Timestamp(40, 1));

After about 30 seconds, the pinned timestamp has not advanced to 40 as expected and the test fails. The symptoms appear to me as if, even though "callbackShouldFail" has been set to false, the dummy callback is still failing and preventing the pinned timestamp from advancing. But that's only a hypothesis.