@alcea Yes! Mastodon since v4.5 has async remote reply fetching, meaning that when looking at the comments of an external post your instance can (and will) contact the external server in the background to fetch new replies.

If new replies are found, they are ingested into mastodon and you will get a small notification at the bottom of indicating there are new replies, with a refresh button.

In the past, you would simply just not see comments your instance didn't run into through it's normal federation pathways, but this is now a thing of the past.