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.
input_weight += weights.GetWeightDiffToMax();
            new_coin_control.SetInputWeight(txin.prevout, input_weight);
        }
    }
    Result result = PreconditionChecks(wallet, wtx, require_mine, errors);
    if (result != Result::OK) {
        return result;
    }
    
    CAmount output_value = 0;
    for (const auto& old_output : wtx.tx->vout) {
        output_value += old_output.nValue;
    }
    old_fee = input_value - output_value;
    
    
    
    std::vector<CRecipient> recipients;
    CAmount new_outputs_value = 0;
    const auto& txouts = outputs.empty() ? wtx.tx->vout : outputs;
    for (const auto& output : txouts) {
        if (!OutputIsChange(wallet, output)) {
            CRecipient recipient = {output.scriptPubKey, output.nValue, false};
            recipients.push_back(recipient);
        } else {
            CTxDestination change_dest;
            ExtractDestination(output.scriptPubKey, change_dest);
            new_coin_control.destChange = change_dest;
        }