In WinUI 2.6, all menu flyout have rounded corners, but TextBox's context menu is not

[roxk]

Describe the bug
Inbox TextBox's context menu doesn't get the rounded corner treatment.

Steps to reproduce the bug

1. Create a blank app. Install WinUI 2.6.0-prerelease.210430001 from nuget.
2. Overwrite the stack panel with the following:

<StackPanel>
    <TextBox PlaceholderText="Input"/>
    <muxc:MenuBar>
        <muxc:MenuBarItem Title="File">
            <MenuFlyoutItem Text="Open"/>
        </muxc:MenuBarItem>
    </muxc:MenuBar>
</StackPanel>

3. Build and see TextBox's context menu is not rounded, but menu bar's menu is rounded.

Expected behavior
TextBox's context menu has rounded corner, consistent with all other menu.

Actual behavior
TextBox's context menu is not rounded.

Screenshots

TextBox

CommandBarFlyout

Version Info

NuGet package version:
[Microsoft.WinUI 2.6.0-prerelease.210430001]

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Note
Just to make sure I didn't miss anything, I tried to reproduce this on the current XAML Control Gallery App on the store and the result is the same. And it seems there is no muxc:TextBox available.

[StephenLPeters]

@chigy FYI

[chigy]

@SavoySchuler FYI

[ghost]

It seems to be #4472 again, which was supposed to be fixed.

[ghost]

Buttons don't get styled properly in the MUXControlsTestApp.

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 5 days.