Parent middleware is called several times

[tearwyx]

Hi, I have this sample code:

const parentRouter = new Router();
const nestedRouter = new Router();

nestedRouter
    .get('/first-nested-route', function *(next) {
        console.log('first nested middleware');

        yield next;
    })

    .get('/second-nested-route', function *(next) {
        console.log('second nested middleware');

        yield next;
    })

    .get('/third-nested-route', function *(next) {
        console.log('third nested middleware');

        yield next;
    });

parentRouter.use('/parent-route',
    function *(next) {
        console.log('parent middleware');

        yield next;
    },
    nestedRouter.routes()
);

app.use(parentRouter.routes());

I requested a /parent-route/first-nested-route and got this debug output:

koa-router GET /parent-route/first-nested-route +5s
koa-router test /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +2ms
koa-router test /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +1ms
koa-router test /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +0ms
koa-router test /parent-route/first-nested-route /^\/parent-route\/first-nested-route(?:\/(?=$))?$/i +0ms
koa-router test /parent-route/second-nested-route /^\/parent-route\/second-nested-route(?:\/(?=$))?$/i +0ms
koa-router test /parent-route/third-nested-route /^\/parent-route\/third-nested-route(?:\/(?=$))?$/i +1ms
koa-router dispatch /parent-route/first-nested-route /^\/parent-route\/first-nested-route(?:\/(?=$))?$/i +0ms
koa-router dispatch /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +1ms
koa-router dispatch /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +0ms
koa-router dispatch /parent-route /^\/parent-route(?:\/(?=$))?(?=\/|$)/i +0ms

And additional output:

parent middleware
parent middleware
parent middleware
first nested middleware

I use the koa-router@5.2.2. Why is parent middleware called several times? Isn't that a bug? Thank you for any inquiries.