<!-- 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,
if (!input.non_witness_utxo && input.witness_utxo.IsNull()) {
            const uint256& txhash = txin.prevout.hash;
            const auto it = mapWallet.find(txhash);
            if (it != mapWallet.end()) {
                const CWalletTx& wtx = it->second;
                
                
                input.non_witness_utxo = wtx.tx;
            }
        }
    }
    
    for (ScriptPubKeyMan* spk_man : GetAllScriptPubKeyMans()) {
        int n_signed_this_spkm = 0;
        TransactionError res = spk_man->FillPSBT(psbtx, sighash_type, sign, bip32derivs, &n_signed_this_spkm);
        if (res != TransactionError::OK) {
            return res;
        }
        if (n_signed) {
            (*n_signed) += n_signed_this_spkm;
        }
    }
    
    complete = true;
    for (const auto& input : psbtx.inputs) {
        complete &= PSBTInputSigned(input);
    }
    return TransactionError::OK;
}
SigningResult CWallet::SignMessage(const std::string& message, const PKHash& pkhash, std::string& str_sig) const