seemingly intentionally, as they don't do this for all titles.

It looks like Viki chose to capitalize the title, full stop. The title remains capitalize in French LES GARÇONS AVANT LES FLEURS and Portuguese GAROTOS EM VEZ DE FLORES. I can't say that I understand why they did it though.

I think, based on this in the context of other examples, that Viki follow how titles are officially capitalised in Japanese networks' programme catalogues.

BOYS OVER FLOWERS is in all caps at both those links; HANA-KIMI is in all caps at https://www.fujicreative.co.jp/for_buyers/more_info/tabid/106/pdid/207/Default.aspx and at https://www.viki.com/tv/38126c-hana-kimi

Contrast this with how https://www.tbscontents.com/en/program/heavenandhell and https://www.viki.com/tv/38388c-heaven-and-hell-soul-exchange use the same conventional capitalisation.

And https://www.fujicreative.co.jp/for_buyers/more_info/tabid/106/pdid/970/Default.aspx https://www.japan-programcatalog.com/en/program/dontcallitmystery and https://www.viki.com/tv/38454c-dont-call-it-mystery all don't capitalise the it in the title, despite it being a noun.

I'm guessing a bit now, but I wonder if, in Japan, Roman-character titles and names are copyrighted as a specific capitalisation, and if the capitalisation is changed it's not considered to be the same name/title.

There is rarely any meaning to the nonstandard capitalisations that Japanese companies use, but they use them and then stick to them consistently.

Western distributors and VOD services tend to ignore the official capitalisations, but Viki is one service which tends to follow them.

Netflix sometimes does, too. https://www.netflix.com/title/81477445 uses the same capitalisation in its logo and text as https://www.tbscontents.com/en/program/japansinks does.