Even with De-elevate on launch flag enabled, it still won't let me to sign into the Edge insider on Windows Sandbox.
Google Chrome stable 79 has no problem under the same circumstances.
@HotCakeX Thanks for letting us know. After chatting with one of the devs, they suggested trying it without running as an admin, as this might be intended behavior.
Fawkes (they/them)Project & Community Manager - Microsoft Edge