If I unlock an encrypted wallet for 359711509437336256 or less seconds, I can sign transactions with it:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336256
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
      "complete": true

But if I unlock it for 359711509437336257 or more seconds, it appears to have unlocked but I can't sign any transactions:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336257
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
    error code: -13
    error message:
    Error: Please enter the wallet passphrase with walletpassphrase first.

I don't know the significance of that number. It's nowhere near a power of 2.

I was trying to unlock the wallet effectively "forever", so put a big number. If I use too big a number (2^63 or more) I get an error message `JSON integer out of range` but for numbers between 359711509437336257 and 2^63-1 inclusive I get no error, but no actual unlocking.
if (fUnlock && !is_locked) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected locked output");
        }
        if (!fUnlock && is_locked) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output already locked");
        }
        outputs.push_back(outpt);
    }
    
    for (const COutPoint& outpt : outputs) {
        if (fUnlock) pwallet->UnlockCoin(outpt);
        else pwallet->LockCoin(outpt);
    }
    return true;
}
UniValue listlockunspent(const JSONRPCRequest& request)
{
    CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() > 0)
        throw std::runtime_error(
            "listlockunspent\n"
            "\nReturns list of temporarily unspendable outputs.\n"
            "See the lockunspent call to lock and unlock transactions for spending.\n"
            "\nResult:\n"
            "[\n"
            "  {\n"
            "    \"txid\" : \"transactionid\",     (string) The transaction id locked\n"