When a personal message is tagged with non-ASCII character tag, the non-ASCII tag can be displayed in the PM Inbox, Sent, Archive and PM Tags list.
However, when clicking the tag under /u/username/messages/tags or /tags, only ASCII tag will return a list of PMs. Clicking on a non-ASCII tag will show “No topics found”.
I am using non-ASCII (Japanese and Chinese) tags sitewide, and haven’t had any problem except in PM.
The issue here is that we are parameterizing the tag name here which strips special characters from strings. This also means that tags with emoji in name will show same behaviour.