Form's FormCornerPreference property value cannot back to Default when change it from a non-Default value

[John-Qiao]

.NET version

.NET SDK 9.0.100-rc.1.24415.1 + build private dlls from winforms repo release/9.0 branch

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

No, this is a new feature Dark Mode: #11857

Issue description

Form's FormCornerPreference property value cannot back to Default when change it from a non-Default value.

Actual result:
The value is 'Round' in textbox control.

Expected result:
The value should be 'Default' in textbox control.

Steps to reproduce

1. Launch the FormCornerPreference-TestApp.zip project in VS and run it.
2. Observe the value in textbox control.

[John-Qiao]

Verified this issue on 9.0.100-rc.2.24419.18 with dlls built from winforms repo of main branch, it was fixed: the value is 'Default' in textbox control now.

[MelonWang1]

@KlausLoeffelmann Verified this issue in the latest .NET 9 RC1 build: 9.0.100-rc.1.24421.39 from https://aka.ms/dotnet/9.0.1xx-rc1/daily/dotnet-sdk-win-x64.exe, it still repro. The value cannot back to 'Default' in textbox control.

[MelonWang1]

Verified the issue with .NET 9.0.100-rc.1.24422.10 test pass build, it still repro, have the same results as above.

[MandiMan]

Verified this issue in the latest .NET9.0 SDK build: 9.0.100-rc.2.24428.9, it still repro, have the same results as above.

[John-Qiao]

Verified the issue with .NET 9.0.100-rc.2.24474.11 test pass build, it still repro, have the same results as above.

[John-Qiao]

Verified this issue with latest .NET 10.0.100-alpha.1.24508.48 from main branch, it was fixed: the value is 'Default' in textbox control now.