<!-- Describe the issue -->

Hi everyone,
We use bitcoin core with hundreds of wallets. The only efficient way we have found to do so is :
1. wait for any bitcoin-cli command to terminate before issuing another one
2. never issue any bitcoin-cli command unless tip update is 1
3. always unload a wallet after loading and using it

Now, we observ that bitcoin core systematically crashes every 20-30 minutes, then automatically restarts. And the problem is, it has to re-sync from about 0.997, which easily takes 15-20 minutes. During that time, the daemon cannot be used.

If we don't load/unload wallets, the daemon is stable and keeps to 1, no issue.

**System information**
Bitcoin Core RPC client version v0.20.0.0-ga62f0ed64f8bbbdfe6467ac5ce92ef5b5222d1bd
Compiled and run on Debian 9, on a VM with 16 mo memory. Launched that way : /bitcoind -daemon -walletrbf -conf=/root/.bitcoin/bitcoin.conf -fallbackfee=0.00001 -rpcworkqueue=128 -txindex=1

**bitcoin.conf :**
server=1
dbcache=16384
par=1
blocksonly=1
maxuploadtarget=137
maxconnections=16
rpcuser=bitcoinrpc
rpcpassword=xxxxxx
rpcallowip=127.0.0.1
testnet=0
txindex=1
walletbroadcast=1
txconfirmtarget=1

Please note that the same behaviour was happening with older releases (from 0.17.x).

Thanks for any help. Please ask for any needed information / testing.
Regards,
const CWalletTx& wtx = it->second;
            
            if (wtx.tx->vout.size() <= outpoint.n) {
                return false;
            }
            
            CInputCoin coin(wtx.tx, outpoint.n, wtx.GetSpendSize(outpoint.n, false));
            nValueFromPresetInputs += coin.txout.nValue;
            if (coin.m_input_bytes <= 0) {
                return false; 
            }
            coin.effective_value = coin.txout.nValue - coin_selection_params.effective_fee.GetFee(coin.m_input_bytes);
            if (coin_selection_params.use_bnb) {
                value_to_select -= coin.effective_value;
            } else {
                value_to_select -= coin.txout.nValue;
            }
            setPresetCoins.insert(coin);
        } else {
            return false; 
        }
    }
    
    for (std::vector<COutput>::iterator it = vCoins.begin(); it != vCoins.end() && coin_control.HasSelected();)
    {
        if (setPresetCoins.count(it->GetInputCoin()))
            it = vCoins.erase(it);
        else
            ++it;
    }