By default, the search ignores German umlauts. Searching for “uber” will also find “über” and so on. This ignorance is not a Relevanssi feature but instead governed by your database. The default database collation WordPress uses (utf8mb4_unicode_ci) ignores all accents, including umlauts.
If you want the search to care about umlauts, you can change the collation. For German, use utf8mb4_german2_ci:
Now the database will not ignore umlauts. Relevanssi also removes accents to match what the database is doing, so you must also undo that. Add this to your site:
remove_filter( 'relevanssi_remove_punctuation', 'remove_accents', 9 );
Once you’ve done these steps, rebuild the index. Now the search should not ignore the umlauts.