Try doing a complete uninstall than reinstall. If that doesn't work and you have another computer install it there and see if the error is the same. You can also try Edge Dev and Canary (in which case if it's also happening it's an issue outside of the browser). Also keep in mind that Edge Beta is only updated every six or so weeks compared to Dev which is every and week and Canary that's about every day. I sand boxed tested Edge Beta, and I didn't have the problem you're having.
Agree, I am also running the same Edge Beta version 78.0.276.8 and went to edge://flags and the styling is broken. It wasn't broken before this update.
