After conversation on irc it appears there's some cases where Bitcoin core will serve feerate information even though it loaded a fairly old fee estimates file and is still syncing the chain. This is pretty dangerous for lightning nodes, at least on legacy channels or pre-package-relay.

The immediate improvement would be to store fee estimates to disk once an hour or so to reduce the chance of having an old file. From there this case could probably be detected and refuse to serve estimates until we sync.
unsigned int bucketIndex2 = shortStats->NewTx(txHeight, (double)feeRate.GetFeePerK());
    assert(bucketIndex == bucketIndex2);
    unsigned int bucketIndex3 = longStats->NewTx(txHeight, (double)feeRate.GetFeePerK());
    assert(bucketIndex == bucketIndex3);
}
bool CBlockPolicyEstimator::processBlockTx(unsigned int nBlockHeight, const CTxMemPoolEntry* entry)
{
    AssertLockHeld(m_cs_fee_estimator);
    if (!_removeTx(entry->GetTx().GetHash(), true)) {
        
        return false;
    }
    
    
    
    int blocksToConfirm = nBlockHeight - entry->GetHeight();
    if (blocksToConfirm <= 0) {
        
        
        LogPrint(BCLog::ESTIMATEFEE, "Blockpolicy error Transaction had negative blocksToConfirm\n");
        return false;
    }
    
    CFeeRate feeRate(entry->GetFee(), entry->GetTxSize());
    feeStats->Record(blocksToConfirm, (double)feeRate.GetFeePerK());
    shortStats->Record(blocksToConfirm, (double)feeRate.GetFeePerK());
    longStats->Record(blocksToConfirm, (double)feeRate.GetFeePerK());
    return true;
}
void CBlockPolicyEstimator::processBlock(unsigned int nBlockHeight,