Setting KeyboardAccelerators on MenuBarItem in Loaded event of user control results in accelerator not working

Nobody has claimed this yet.

### Describe the bug

See attached repro project. Notice how Ctrl+Shift+D works while Ctrl+Shit+F does not.

[App14.zip](https://github.com/user-attachments/files/24914009/App14.zip)

### Why is this important?

We ran into this after refactoring and suddenly no shortcuts working anymore.

### Steps to reproduce the bug

1. Create MenuBar inside a UserControl
2. Add MenuBarItem in loaded event, add keyboard accelerators
3. Hit keys of accelerator on keyboard, observe it not triggering.

In the case of the repro project, Ctrl+Shift+D works while Ctrl+Shift+F does not.

**If the menubar is open, the shortcut Ctrl+Shift+F does work, if it is closed it does not.**

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.8.3: 1.8.251106002

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start with the attached App14.zip repro and follow the MenuBar/UserControl path where the MenuBarItem and keyboard accelerators are added in the Loaded event. Reproduce the difference between Ctrl+Shift+D and Ctrl+Shift+F with the menubar closed and open. Done means the accelerator added during Loaded triggers while the menubar is closed.

Written by the indexing model from the issue text.