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.
RenameThread("bitcoin-scriptch");
    scriptcheckqueue.Thread();
}
VersionBitsCache versionbitscache;
int32_t ComputeBlockVersion(const CBlockIndex* pindexPrev, const Consensus::Params& params)
{
    LOCK(cs_main);
    int32_t nVersion = VERSIONBITS_TOP_BITS;
    for (int i = 0; i < (int)Consensus::MAX_VERSION_BITS_DEPLOYMENTS; i++) {
        ThresholdState state = VersionBitsState(pindexPrev, params, static_cast<Consensus::DeploymentPos>(i), versionbitscache);
        if (state == ThresholdState::LOCKED_IN || state == ThresholdState::STARTED) {
            nVersion |= VersionBitsMask(params, static_cast<Consensus::DeploymentPos>(i));
        }
    }
    return nVersion;
}
/**
 * Threshold condition checker that triggers when unknown versionbits are seen on the network.
 */
class WarningBitsConditionChecker : public AbstractThresholdConditionChecker