Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
{
    AssertLockHeld(cs_main);
    
    
    
    
    
    
    flags = std::max(flags, 0);
    
    
    
    
    
    
    const int nBlockHeight = chainActive.Height() + 1;
    
    
    
    
    
    const int64_t nBlockTime = (flags & LOCKTIME_MEDIAN_TIME_PAST)
                             ? chainActive.Tip()->GetMedianTimePast()
                             : GetAdjustedTime();
    return IsFinalTx(tx, nBlockHeight, nBlockTime);
}
bool TestLockPointValidity(const LockPoints* lp)