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.
"\nResult:\n"
            "\"txid\"                 (string) The transaction id.\n"
            "\nExamples:\n"
            "\nSend 0.01 " + CURRENCY_UNIT + " from the default account to the address, must have at least 1 confirmation\n"
            + HelpExampleCli("sendfrom", "\"\" \"1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd\" 0.01") +
            "\nSend 0.01 from the tabby account to the given address, funds must have at least 6 confirmations\n"
            + HelpExampleCli("sendfrom", "\"tabby\" \"1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd\" 0.01 6 \"donation\" \"seans outpost\"") +
            "\nAs a json rpc call\n"
            + HelpExampleRpc("sendfrom", "\"tabby\", \"1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd\", 0.01, 6, \"donation\", \"seans outpost\"")
        );
    ObserveSafeMode();
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK2(cs_main, pwallet->cs_wallet);
    std::string strAccount = AccountFromValue(request.params[0]);
    CTxDestination dest = DecodeDestination(request.params[1].get_str());
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    }
    CAmount nAmount = AmountFromValue(request.params[2]);
    if (nAmount <= 0)
        throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount for send");
    int nMinDepth = 1;
    if (!request.params[3].isNull())
        nMinDepth = request.params[3].get_int();
    CWalletTx wtx;
    wtx.strFromAccount = strAccount;
    if (!request.params[4].isNull() && !request.params[4].get_str().empty())
        wtx.mapValue["comment"] = request.params[4].get_str();