Social media users are increasingly using unusual Unicode chars to stylize their display name.
eg 𝔅, 𝓰
On my 7.1.1 device these display as ▯▯▯▯▯ which is a bit of an annoyance.
I've installed the latest 8.1 version of Google Roboto along with other fonts as the system font. The changes apply ok in launcher and apps but the problem chars still appear as the glyph box.
In testing the same usernames on other devices (such as a P20Pro) there is no issue.