Ubuntu Thin font don't shuw up in list

Hello there I'm new, I got a Kindle basic since 2015 but i started really using it just last year. I both bought books and sideloaded some good ol' public domain titles (that literally and literary open a world for me)

So, to the question: I use ubuntu typeface family but since some month ago, i believe, i cannot choose the Thin font no matter what format or from what source the book is except when Ubuntu Thin is the only font present in the custom font folder. Do you happen to have a clue?

THANKS in advance :)