Indeterminate `HashSet::contains` behavior

I tried this code:

use std::borrow::Borrow;
use std::collections::HashSet;

#[derive(PartialEq, Eq, Hash)]
struct Label(pub [u8; 7]);

impl Label {
    pub fn new(x: &str) -> Self {
        let mut bytes = x.bytes();
        Label(std::array::from_fn(|_| bytes.next().unwrap()))
    }
}

impl Borrow<str> for &Label {
    fn borrow(&self) -> &str {
        println!("───────────────────────────────────────────────────────────────────");
        std::str::from_utf8(&self.0).unwrap()
    }
}

impl Borrow<[u8]> for &Label {
    fn borrow(&self) -> &[u8] {
        &self.0
    }
}

fn main() {
    let labels = [
        Label::new("label01"),
        Label::new("label02"),
        Label::new("label03"),
    ];
    let mut hs = HashSet::new();
    for i in 0..labels.len() {
        hs.insert(&labels[i]);
    }

    println!("Contains -> {}", hs.contains(b"label01" as &[u8]));
    println!("Contains -> {}", hs.contains("label01"));
}

I included an implementation of Borrow<[u8]> for &Label as a sanity check, but the main focus here is Borrow<str> for &Label.

I expected to see this happen:

For every run to print the long line "────", triggered by the .contains() call in the last line of main().

Instead, this happened:

The long line "────" is only printed on certain runs. It prints at about one in every 30 runs of cargo run.

Meta

rustc --version --verbose:

rustc 1.80.0 (051478957 2024-07-21)
binary: rustc
commit-hash: 051478957371ee0084a7c0913941d2a8c4757bb9
commit-date: 2024-07-21
host: aarch64-apple-darwin
release: 1.80.0
LLVM version: 18.1.7

Start by reproducing the supplied Rust program and inspecting the HashSet::contains entry point, especially how Borrow and Borrow<[u8]> are selected for the stored references. Determine why the str lookup is intermittent, then add coverage for repeated lookups and verify that the expected Borrow behavior is consistent.