When it works

If a class component is exported from a module that also exports a functional component.

Sample:

import React from "react";

export class Bar1 extends React.Component {
    render() {
        return <span>Barx</span>
    }
}

export function DummyBarFc() {
    return <span>dummy</span>
}

When it does not work

If a class component is exported from a module that exports no functional component.

Sample:

import React from "react";

export class Bar1 extends React.Component {
    render() {
        return <span>Barx</span>
    }
}

// export function DummyBarFc() {
//     return <span>dummy</span>
// }

Why it does not work

The refresh runtime is only included

if the code of module matches

However, the pattern is not generated by SWC for class components, as fast refresh does not preserve their state (see swc-project/swc#4431 (comment)).
So a module with no exported functional component is not handled by HMR.

(Note: I will provide a PR.)