Unicode /special characters are duplicated with an @ symbol

[gunchleoc]

Whenever I post a non-ASCII character, like e.g. ü, it gets turned into <character>@<character>, e.g. ü will turn into ü@ü, with everything after & including the @ turned into a link to a username.

Example post: https://pixelfed.de/i/web/post/486500870917754954

Contains:

fü[@ür](https://pixelfed.de/i/web/username/%C3%BCr)den Link

I post using the Web UI

[gunchleoc]

This also happens when I post with a bot that uses the Python Mastodon.py library.

Display is also wrong in the Android Pixelfed app and in Tusky, so the bug must be server side.

[gunchleoc]

Possibly the same issue: #4481