Hello everyone.
I own a Mi 9T (International version of Redmi K20, which have a Global ROM) and when I was on stock ROM, Widevine L1 was available without any trouble. But I've switched today to xiaomi.eu and L1 disappeared in favour of L3.
So I think Widevine is included on Mi 9T and therefore not on K20, so that's why we couldn't find any certificates in the right section (because xeu rom is based under K20).