1) Open https://beta.musicbrainz.org/series/create and set language to Estonian, Finnish, German, or some other that has characters outside plain ASCII.
2) Choose something from the "Type" or "Ordering Type" field.
3) Observe the glory of ä, ö, ü - instead of the more usual ä, ö, ü. (Affects other non-ASCII characters as well, see e.g. French for those.)

Confirmed by Freso in IRC.

I vaguely remember seeing this elsewhere, too, but don't remember where exactly. I'll post a comment when I find something more.