<!-- 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,
for (const auto& entry : mapWallet) {
        
        const CWalletTx &wtx = entry.second;
        if (wtx.m_confirm.status == CWalletTx::CONFIRMED) {
            
            for (const CTxOut &txout : wtx.tx->vout) {
                
                for (const auto &keyid : GetAffectedKeys(txout.scriptPubKey, *spk_man)) {
                    
                    auto rit = mapKeyFirstBlock.find(keyid);
                    if (rit != mapKeyFirstBlock.end() && wtx.m_confirm.block_height < rit->second->block_height) {
                        rit->second = &wtx.m_confirm;
                    }
                }
            }
        }
    }
    
    for (const auto& entry : mapKeyFirstBlock) {
        int64_t block_time;
        CHECK_NONFATAL(chain().findBlock(entry.second->hashBlock, FoundBlock().time(block_time)));
        mapKeyBirth[entry.first] = block_time - TIMESTAMP_WINDOW; 
    }
}
/**
 * Compute smart timestamp for a transaction being added to the wallet.
 *
 * Logic:
 * - If sending a transaction, assign its timestamp to the current time.