Lowercase Internationalisation not Working

[Chaoswrd]

Operating system: Windows 10 pro
Config file:
filter { mutate { add_field => { "[location][lat]" => "%{latitude}" } add_field => { "[location][lon]" => "%{longitude}" } lowercase => [ "first_name", "last_name", "country", "street", "postalcity", "region" ] } }

Samples:
first_name: ÅKE => Åke
postalcity: BJÖRNEBORG => bjÖrneborg

Comment:
It seems that internationalisation is not working. Tested uppercase and lowercase with ruby 2.4.2 and it seems to handle internationalisation. When running logstash it won't lowercase those letters though it does lowercase everything else. Is lowercase set to ascii?