<!-- 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 (result.status == ScanResult::FAILURE) {
            int64_t time_max;
            CHECK_NONFATAL(chain().findBlock(result.last_failed_block, FoundBlock().maxTime(time_max)));
            return time_max + TIMESTAMP_WINDOW + 1;
        }
    }
    return startTime;
}
/**
 * Scan the block chain (starting in start_block) for transactions
 * from or to us. If fUpdate is true, found transactions that already
 * exist in the wallet will be updated.
 *
 * @param[in] start_block Scan starting block. If block is not on the active
 *                        chain, the scan will return SUCCESS immediately.
 * @param[in] start_height Height of start_block
 * @param[in] max_height  Optional max scanning height. If unset there is
 *                        no maximum and scanning can continue to the tip
 *
 * @return ScanResult returning scan information and indicating success or
 *         failure. Return status will be set to SUCCESS if scan was
 *         successful. FAILURE if a complete rescan was not possible (due to
 *         pruning or corruption). USER_ABORT if the rescan was aborted before
 *         it could complete.
 *
 * @pre Caller needs to make sure start_block (and the optional stop_block) are on
 * the main chain after to the addition of any new keys you want to detect
 * transactions for.
 */
CWallet::ScanResult CWallet::ScanForWalletTransactions(const uint256& start_block, int start_height, Optional<int> max_height, const WalletRescanReserver& reserver, bool fUpdate)