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<!-- Describe the issue -->
Have two wallets used by this instance of bitcoind. The default wallet has balance 0.02861762.
Initially reported balance is correct but after unloading the wallet and then loading it again, the balance is wrong. Neither of the wallets have the reported balance.
<!--- What behavior did you expect? -->
I expected the balance to be the same as initially reported.
```
btc@ubuntu:~$ bitcoin-cli  -getinfo
{
  "version": 170100,
  "protocolversion": 70015,
  "walletversion": 159900,
  "balance": 0.02861762,
...

btc@ubuntu:~$ bitcoin-cli  loadwallet wallet2
btc@ubuntu:~$ bitcoin-cli unloadwallet ""
btc@ubuntu:~$ bitcoin-cli loadwallet ""
btc@ubuntu:~$ bitcoin-cli unloadwallet wallet2
btc@ubuntu:~$ bitcoin-cli  -getinfo
{
  "version": 170100,
  "protocolversion": 70015,
  "walletversion": 159900,
  "balance": 0.01012607,
...
```
The balance 0.01012607 is wrong and I have no idea where that number came from.
Restarting the process results in correct balance again.
<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->
ubuntu package for bionic
<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

<!-- For the GUI-related issue on Linux provide names and versions of a distro, a desktop environment and a graphical shell (if relevant). -->

<!-- Any extra information that might be useful in the debugging process. -->
<!--- This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred. -->

* Private version of AddWatchOnly method which does not accept a
     * timestamp, and which will reset the wallet's nTimeFirstKey value to 1 if
     * the watch key did not previously have a timestamp associated with it.
     * Because this is an inherited virtual method, it is accessible despite
     * being marked private, but it is marked private anyway to encourage use
     * of the other AddWatchOnly which accepts a timestamp and sets
     * nTimeFirstKey more intelligently for more efficient rescans.
     */
    bool AddWatchOnly(const CScript& dest) override EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    interfaces::Chain& m_chain;
    
    WalletLocation m_location;
    
    std::unique_ptr<WalletDatabase> database;
    /**
     * The following is used to keep track of how far behind the wallet is
     * from the chain sync, and to allow clients to block on us being caught up.
     *
     * Note that this is *not* how far we've processed, we may need some rescan
     * to have seen all transactions in the chain, but is only used to track
     * live BlockConnected callbacks.
     */
    uint256 m_last_block_processed;
public:
    /*
     * Main wallet lock.
     * This lock protects all the fields added by CWallet.
     */
    mutable CCriticalSection cs_wallet;