你的代码中有两个问题：
1.错误的命名空间（您已经修复了）
1.未在代码隐藏中修复继承的类
下面是自定义控件的完整代码，使用Grid作为根元素是没有问题的

<?xml version="1.0" encoding="utf-8" ?>
<Grid
  x:Class="MauiApp2.Controls.MyActivityIndicator"
  xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
  xmlns:local="clr-namespace:MauiApp2.Controls"
  HorizontalOptions="FillAndExpand"
  RowDefinitions="*,*">
  <Label
    Grid.Row="0"
    FontAttributes="Bold"
    HorizontalOptions="CenterAndExpand"
    Text="{Binding Source={RelativeSource AncestorType={x:Type local:MyActivityIndicator}}, Path=Text, Mode=OneWay}" />
  <ActivityIndicator
    Grid.Row="1"
    HorizontalOptions="CenterAndExpand"
    IsRunning="{Binding Source={RelativeSource AncestorType={x:Type local:MyActivityIndicator}}, Path=IsRunning, Mode=OneWay}" />
</Grid>

namespace MauiApp2.Controls;

public partial class MyActivityIndicator : Grid // <-- you have to change that manually
{
    public MyActivityIndicator()
    {
        InitializeComponent();
    }

    public static BindableProperty TextProperty = BindableProperty.Create( nameof( Text ), typeof( string ), typeof( MyActivityIndicator ), default( string ), BindingMode.OneWay );
    public string Text { get => (string)GetValue( TextProperty ); set => SetValue( TextProperty, value ); }

    public static BindableProperty IsRunningProperty = BindableProperty.Create( nameof( IsRunning ), typeof( bool ), typeof( MyActivityIndicator ), false, BindingMode.OneWay );
    public bool IsRunning { get => (bool)GetValue( IsRunningProperty ); set => SetValue( IsRunningProperty, value ); }
}

错误消息的根本原因是xaml中来自maui和xamarin的混合命名空间。
在清理命名空间使其只包含maui命名空间之后，这些控件被识别出来。
但是错误“The attachable property 'Row' was not found in type 'Grid'.”仍然存在。似乎你不能使用Grid作为根元素。在ContentView控件中包含网格后，一切正常。

<ContentView xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="MyProject.Controls.MyActivityIndicator">
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Label x:Name="LoadingLabel" FontAttributes="Bold" Grid.Row="0" HorizontalOptions="CenterAndExpand" />
    <ActivityIndicator x:Name="ActivityIndicator" Grid.Row="1" HorizontalOptions="CenterAndExpand" />
</Grid>