The solution mentioned here fixed the issue for me: https://learn.microsoft.com/en-us/answers/questions/805883/is-there-an-alternative-to-generate-a-class-diagra
Creating the XML file, pasting the shown markup, closing and reopening gives you a working class diagram. I have saved off a copy of this file to add whenever I need a class diagram in .Net Core. I am surprised this issue has not been resolved in over a year since it was reported - but the work around is pretty simple.