As summarized in the following article, there are some issues with full-text search in Japanese in Omeka S with the default settings.

Omeka Sの日本語による全文検索の注意点とMroonga searchモジュール - デジタルアーカイブシステムの技術ブログ
この記事は、以下に移転しました。 https://ldas.jp/ja/posts/hatena-2022-03-07-083004/ ブックマークや被リンクをお持ちの場合は、移転先への変更をお願いします。今後の更新はすべて移転先で行います。
デジタルアーカイブシステムの技術ブログ

In the above article, I introduced "Workaround 2" as a simple solution to this issue.

This time, I created a repository that applies this workaround to "Foundation S," one of the themes for Omeka S.

GitHub - nakamura196/foundation-s: An Omeka S theme based off ZURB Foundation
An Omeka S theme based off ZURB Foundation. Contribute to nakamura196/foundation-s development by creating an account on GitHub.
GitHub

I hope this is helpful for those using the "Foundation S" theme who are experiencing issues with Japanese search.