<!-- 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,
bool reorg = false;
        if (chain().findBlock(block_hash, FoundBlock().data(block)) && !block.IsNull()) {
            LOCK(cs_wallet);
            next_block = chain().findNextBlock(block_hash, block_height, FoundBlock().hash(next_block_hash), &reorg);
            if (reorg) {
                
                
                
                
                result.last_failed_block = block_hash;
                result.status = ScanResult::FAILURE;
                break;
            }
            for (size_t posInBlock = 0; posInBlock < block.vtx.size(); ++posInBlock) {
                SyncTransaction(block.vtx[posInBlock], {CWalletTx::Status::CONFIRMED, block_height, block_hash, (int)posInBlock}, fUpdate);
            }
            
            result.last_scanned_block = block_hash;
            result.last_scanned_height = block_height;
        } else {
            
            result.last_failed_block = block_hash;
            result.status = ScanResult::FAILURE;
            next_block = chain().findNextBlock(block_hash, block_height, FoundBlock().hash(next_block_hash), &reorg);
        }
        if (max_height && block_height >= *max_height) {
            break;
        }
        {
            if (!next_block || reorg) {