Rust FnMut closure updates a var but doesn't stick

Question

I have this rust code where I am trying to implement observability. And, in my closure add_foo_listener I simply update the called variable and then assert it it's called.

This code compiles, and the closure is invoked, as is evidenced by the println, but the called variable doesn't stick. Everything I read says I should be using FnMut which, I believe I am. But, I don't understand why this boolean isn't being updated

    #[derive(Debug, Clone, Hash, PartialEq, Eq)]
    struct Foo {
        baz: u32
    }
    struct Holder {
        pub foo_observers: Vec<Box<dyn FnMut(Foo)>>,
    }

    impl Holder {

        pub fn add_foo_listener<CB: 'static + FnMut(Foo) + FnMut(Foo)>(&mut self, on_foo: CB) -> &mut Self {
            self.foo_observers.push(Box::new(on_foo));
            self
        }

        pub fn notify_foo_event(&mut self, foo: Foo) -> &mut Self {
            self.foo_observers
                .iter_mut()
                .for_each(|notify| notify(foo.clone()));
            self
        }
    }
    #[test]
    fn test_foo() -> Result<(), String> {
        let mut holder = Holder{
            foo_observers: Vec::new()
        };
        let mut called: bool = false;
        let foo = Foo{
            baz : 10
        };

        holder.add_foo_listener( move |f: Foo| {
            assert_eq!(f,foo);
            println!("Listener called");
            called = true;
        }).notify_foo_event(foo);
        assert!(called); // called is still false
        Ok(())
    }

Output:

---- notifier::test::test_foo stdout ----
Listener called
thread 'notifier::test::test_foo' panicked at 'assertion failed: called', foo.rs
assert!(called); 
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace