If you're viewing a thread in Mastodon, try waiting 5 - 10 seconds to give it time to load in all the replies from all the other servers. When it's done loading it will show a little pop up message "Show more replies" or similar, click on the message to view all replies in the thread.
In the old days it didn't do this and threads looked different on every server, but since Mastodon 4.5.0 it fetches all replies in a thread, so threads now look the same on every server.