Still looking for answers? I was too. None of the Powershell fixes worked for me.
I finally found my fix here: https://www.winhelponline.com/blog/need-new-app-open-windows-defender-link/#download
The downloads listed are not hosted by Microsoft but they are properly signed my Microsoft and scanned clean for me.
Basically I had to redownload and reinstall both Microsoft.UI.Xaml (Microsoft.SecHealthUI_8wekyb3d8bbwe.appx) and Microsoft.SecHealthUI (Microsoft.UI.Xaml.2.4_2.42007.9001.0_x64__8wekyb3d8bbwe.appx)... afterward the Security app opened as it should.