Strange. I had this issue with every username I tried. as unicode characters I only had German umlauts like äöü. Another example was: Waegi back to Wägi.
This issue first came up in the phpBB3 import to Discourse script, which was the reason for me to rename all umlaut users in phpBB and then restart the import script. For sure I wanted the users in Discourse again with their original username.