MainWindow.axaml.cs

using Avalonia.Collections;
using Avalonia.Controls;
using DynamicData;
using DynamicData.Binding;
using System;
using System.Collections.ObjectModel;
using System.Linq;
using System.Reactive.Subjects;

namespace DataGridDebug
{
    internal record Row(int Value, string Group);

    public partial class MainWindow : Window
    {
        private readonly BehaviorSubject<Func<Row, bool>> _filter = new(_ => true);

        public MainWindow()
        {
            InitializeComponent();
            
            ObservableCollection<Row> rows = new(Enumerable.Range(1, 50).Select(v => new Row(v, "Group")));
            rows
                .ToObservableChangeSet()
                .Filter(_filter)
                .Bind(out ReadOnlyObservableCollection<Row> filteredRows)
                .Subscribe();

            var groupedSource = new DataGridCollectionView(filteredRows, true, true);
            DataGridPathGroupDescription group = new("Group");
            groupedSource.GroupDescriptions.Add(group);
            TestDataGrid.ItemsSource = groupedSource;
        }

        private void Button_Click(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
        {
            if (int.TryParse(TestTextBox.Text ?? string.Empty, out var count))
            {
                _filter.OnNext(r => r.Value <= count);
            }
        }
    }
}