How to reproduce:
Thank you for reporting this problem, I will investigate.
I've realised the issue is not the characters but the number of characters entered. beorg only performs the search is 3 or more characters are entered. This minimum length isn't helpful for languages such as Chinese. What I'm going to do is add a setting so a user can change this. The reason for the setting is performance when searching using the UI. You'll see this new setting in the next update.
Seems that there is still no such option in version 2.15.0(129)
This is a setting that needs to be added to your init.org - for example
(set! filter-min-chars 2)
If you aren't familiar with using an init.org for advanced configuration of beorg please review https://beorg.app/learning/excluding-tasks-from-specific-files-for-the-agenda-and-todo-lists/