if TMP font asset dosen't contain the character, it will show 口.
Once setup font fallback, text can't reveal normally.

video
https://www.youtube.com/watch?v=5Dxl31qIU68

This was fixed in v1.16 (available in #download on Discord) and latest TMPro update (install the latest version in package manager, make sure to use Unity 2019.4.24 or later).