If I create an RBF transaction using the Qt GUI but specify my own custom change address, bumpfee doesn't work because it claims it can't find the change address:

    "Transaction does not have a change output"

This is because none of the outputs satisfy

    pWallet->IsChange(...)

I see there's a "TODO" commented in `CWallet::IsChange()` in `src/wallet/wallet.cpp`:

    // TODO: fix handling of 'change' outputs. The assumption is that any
    // payment to a script that is ours, but is not in the address book
    // is change.

I tend to put all my addresses in the address book whether I'm going to use them as change or not. I guess if the wallet was capable of creating its own SegWit change addresses I'd stop using custom change addresses, but currently it always seems to send change to a legacy address if I don't manually specify a change address.
{
    return
    {
        {"", RPCArg::Type::OBJ_USER_KEYS, RPCArg::Optional::OMITTED, "",
            {
                {"address", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "A key-value pair. The key (string) is the bitcoin address,\n"
                         "the value (float or string) is the amount in " + CURRENCY_UNIT + ""},
            },
        },
        {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
            {
                {"data", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "A key-value pair. The key must be \"data\", the value is hex-encoded data"},
            },
        },
    };
}
static RPCHelpMan bumpfee_helper(std::string method_name)