When the user specifies the channel name with 2-byte characters, it is noted with red letters “Name must be lowercase alphanumeric characters of two or more letters”.
Currently it is once created in alphabetic characters and modified to 2-byte characters after channel generation.
2-byte characters can be used with PC version, PC web browser, smartphone web browser without problem.
Mattermost 5.8.0, iOS apps 1.16.1, iOS 12.1.4
Jesse: We're either not enforcing the length consistently for multi-byte characters (e.g. between creation and rename), or we're incorrectly applying it at creation time.