Woocommerce dependant attributes BUG?

hi guys, i’m having a really weird behavior using woocommerce variable products. I was looking for a way to have dependant attributes on my variable product, for example i have a “line” attribute and a “color” attribute. Colors belongs to specific lines so i wanted to show only the colors that belong to the specific line. At the end i’ve created a variation for each line and color and it was filtering the products correctly, but if i add more that 30 variations then the attributes are not filetered anymore. It is pretty weird because it works until 30, after that number, it stop working.

see examples here:
https://postimg.org/image/3lc84zphv/ => this product has 31 variations
https://postimg.org/image/6pwob7619/ => this product has 30 variations

hope you understand and of course hope you can give me any hint of what’s happening.