Full-text search in Japanese does not work properly with Omeka S’s standard functionality. The following article explains the details of the issue:
https://nakamura196.hatenablog.com/entry/2022/03/07/083004
In the article above, I presented several workarounds, and I have now created a module that encompasses those solutions:
https://github.com/nakamura196/Omeka-S-module-FixCjkSearch
While this is not a fundamental fix, I hope it proves useful when working with Japanese materials in Omeka S.