I can only suggest trying it again...I just tried it and the fills were suitable and sounded correct.

If all the rest of the style (Intros, Endings etc) are straight 8ths and the fills are still triplets, it can't be changing the whole style.

Make sure you are first pressing a style Variation button (A,B,C, or D) and then selecting ALL at the opening SECTION screen (bottom right) before you set the ORIGINAL BEAT to "12" and BEAT CONVERTOR to "8" and press EXECUTE.

Ian