Why does this code no longer compile when the concept is made variadic?

Question

Let's define the following templates:

template<typename> concept C_one = true;
template<typename...> concept C_many = true;

template<bool> struct S { };

Now, let's try to use them.
This line compiles:

template<typename T = S<C_one<int>>> void f() { }

And this one does not:

template<typename T = S<C_many<int>>> void f() { }

The error I'm getting is:

error: type/value mismatch at argument 1 in template parameter list for 'template<bool <anonymous> > struct S'

I noticed that wrapping C_many<int> in parentheses fixes the issue, but what is the reason for such seemingly inconsistent syntax? I think the error message isn't very helpful either.
Also, I am aware that this might be a pretty contrived example, because I tried to create a minimal reproducible example.