Usually, Mastodon only shows the first part (or "handle") of a Fediverse address in posts.
However, if there are two accounts with the same first part, Mastodon shows the accounts' full addresses including their server names.
This is deliberate, it's to avoid confusion when two different accounts with identical handles are being discussed in the same post.
For example the libre gardening organisation Open Source Gardens has a main account at @opensourcegardens@chaos.social and a video account at @opensourcegardens@tube.tchncs.de which both have the same handle. As you can see, Mastodon automatically displays their full Fediverse addresses.
