If you refer to this answer, at the bottom in bold the user points out that you require double slashes in the scope.

scopes: [
    "https://service.flow.microsoft.com//User", // Note the double slash before User
]

I came across your question first, had a similar issue and had a single slash in my scope, then I found that answer and added the slash (whether using .default or User) and it started working.

(I was going to add this as a comment, not an answer, but I need more reputation first, hopefully this qualifies as an answer)