***UPDATE:***

Turns out the issue was caused not by testnet, but by having 2 wallets @ryanofsky found out !

----------

Step to reproduce:

This work on regtest just fine:
```
bitcoind -wallet=test -regtest -rpcpassword=abc -rpcuser=abc
```

Try to make a batch RPC request to /wallet/test:

```
[{"jsonrpc":"1.0","id":1,"method":"getbalance","params":["*"]}]
```
You get the expected result.

However; same things with testnet:
```
bitcoind -wallet=test -testnet -rpcpassword=abc -rpcuser=abc
```

Result:

"Wallet file not specified (must request wallet RPC through /wallet/<filename> uri-path)."

This problem is limited to batched RPC request on testnet as just making the exact same request without the '[' and ']' works fine. It works also fine on regtest, which is crazy. I will look into later.
{
    if (!value.isNum() && !value.isStr())
        throw JSONRPCError(RPC_TYPE_ERROR, "Amount is not a number or string");
    CAmount amount;
    if (!ParseFixedPoint(value.getValStr(), 8, &amount))
        throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount");
    if (!MoneyRange(amount))
        throw JSONRPCError(RPC_TYPE_ERROR, "Amount out of range");
    return amount;
}
uint256 ParseHashV(const UniValue& v, std::string strName)