Finnish months has wrong case suffixes

Months have wrong case suffixes when using Finnish translations. For example January is 'Tammikuu' in Finnish, and if you open up Moodle forum or see some other deadlines, and you want to say that the deadline is 20th of January is should be in Finnish "20. Tammikuuta" but now it is "20. Tammikuutata". So there is one extra syllable.

For example in this forum the timestamp is "tiistaina, 21. maaliskuutata 2023, 00:00" when it should be "tiistaina, 21. maaliskuuta 2023, 00:00".
https://moodle.org/security/index.php?lang=fi

This is similar than if someone writes the deadline to be on Tuesday 21st of of March.

Is there some bug that adds double case suffix since this is not language pack issue (according to e.g. Translation FAQ])? Locale shows that I'm using Finnish as should and if the issue is visible on other platforms I assume that this is caused by some coding error.