VLC cannot correctly display karaoke subtitling in ass files when the speed is not normal

When I watched videos with an ass subtitle with karaoke subtitling in the normal speed(1.00x), the subtitle was displaying well. But when I switch the speed higher(e.g. 2.00x), the karaoke was slower than the sound track; when I switch it lower(e.g. 0.50x), the karaoke was faster than the sound track. I guess that the selected audio (and video) speed did nothing to the speed of playing the subtitle (at least the karaoke mode), but the three speeds should be the same.