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.
fCheckForPruning = false;
            }
            if (!setFilesToPrune.empty()) {
                fFlushForPrune = true;
                if (!fHavePruned) {
                    pblocktree->WriteFlag("prunedblockfiles", true);
                    fHavePruned = true;
                }
            }
        }
        nNow = GetTimeMicros();
        
        if (nLastWrite == 0) {
            nLastWrite = nNow;
        }
        if (nLastFlush == 0) {
            nLastFlush = nNow;
        }
        if (nLastSetChain == 0) {
            nLastSetChain = nNow;
        }
        int64_t nMempoolSizeMax = gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
        int64_t cacheSize = pcoinsTip->DynamicMemoryUsage();
        int64_t nTotalSpace = nCoinCacheUsage + std::max<int64_t>(nMempoolSizeMax - nMempoolUsage, 0);
        
        bool fCacheLarge = mode == FlushStateMode::PERIODIC && cacheSize > std::max((9 * nTotalSpace) / 10, nTotalSpace - MAX_BLOCK_COINSDB_USAGE * 1024 * 1024);
        
        bool fCacheCritical = mode == FlushStateMode::IF_NEEDED && cacheSize > nTotalSpace;
        
        bool fPeriodicWrite = mode == FlushStateMode::PERIODIC && nNow > nLastWrite + (int64_t)DATABASE_WRITE_INTERVAL * 1000000;