Sorry for not including instructions on darts, but the objective is to perfectly score the amount of points to take your total to exactly 0. If you go over this amount your score will reset to whatever it last was before it went below 100. There is an item you can obtain that will temporarily remove this rule for 1 dart throw :) .
If you already knew all of this and that's not what is happening then my apologies that is an unknown bug and something we'll have to fix.
Thank you!