@GrecKo Indeed, which is why I've had to resort to always emitting dataChanged and never returning false.

I was surprised, but it does return the TextInput to the model's value. Full code:

Main.qml

import QtQuick

Window {
    visible: true
    required property QtObject m

    ListView {
        anchors.fill: parent
        model: m
        delegate: TextInput {
            text: display
            onEditingFinished: display = text
        }
    }
}

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QStandardItemModel>

class Model : public QStandardItemModel {
public:
    bool setData(const QModelIndex &index, const QVariant &value, int role) override {
        auto updated = (role == Qt::ItemDataRole::DisplayRole) ? value.toString().toUpper() : value;
        if (role == Qt::ItemDataRole::DisplayRole && updated.toString().contains('!')) {
            return false;
        }

        return QStandardItemModel::setData(index, updated, role);
    }
};

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    Model model;
    model.appendRow(new QStandardItem("something"));

    QQmlApplicationEngine engine;
    engine.setInitialProperties({ {"m", QVariant::fromValue(&model)} });
    engine.loadFromModule("untitled1", "Main");

    return app.exec();
}

onEditingFinished: m.setData(m.index(index, 0), text)

No need to call setData either in 6.9, you can do it with onEditingFinished: model.display = text.

Indeed. Simplifying further:

ListView {
    anchors.fill: parent
    model: m
    delegate: TextInput {
        text: display
        onEditingFinished: display = text
    }
}

bool setData(const QModelIndex &index, const QVariant &value, int role) override {
    auto updated = (role == Qt::ItemDataRole::DisplayRole) ? value.toString().toUpper() : value;
    if (role == Qt::ItemDataRole::DisplayRole && updated.toString().contains('!')) {
        return false;
    }
   return QStandardItemModel::setData(index, updated, role);
}

onEditingFinished: m.setData(m.index(index, 0), text)

No need to call setData either in 6.9, you can do it with onEditingFinished: model.display = text.

For earlier versions, the delegate can call the model's setData(), which can emit dataChanged() even if the change attempt is rejected. This works for me with Qt 6.9:
QML:

ListView {
    model: m
    delegate: TextInput {
        text: display
        onEditingFinished: m.setData(m.index(index, 0), text)
    }
}

C++

bool setData(const QModelIndex &index, const QVariant &value, int role) override
{
    auto updated = (role == Qt::ItemDataRole::EditRole) ?
                   value.toString().toUpper() : value;
    if (role == Qt::ItemDataRole::EditRole && updated.toString().contains('!')) {
        emit dataChanged(index, index, { Qt::ItemDataRole::DisplayRole });
        return false;
    }
    return QStandardItemModel::setData(index, updated, role);
}

Another option is the Qt Widgets model-view pattern, with a separate editor that is displayed when the delegate instance is activated. On completion of editing, the editor calls setData(). The delegate itself remains bound to model data, and is unaware of intermediate or invalid edits. Exchange more code in the delegate for less code in the model.