This is not a bug, some of the Mines are fakes and will explode once you defuse them to a certain point. You need to look at the Mines themselves before you move into the circle. If a Mine is crackling with Arc energy then it is fake and will explode if you try to disarm it; simply let these Mines count themselves down and move on to look for others that don’t exhibit these signs. Note that the Dismantle Mines objective succeeds if all the actual Mines are defused, regardless of how many fakes detonate due to inattentive players. (΄◉◞౪◟◉｀)