The objective wouldn't be so bad, if the mines wouldn't randomly explode while you are disarming them, despite still having time left.
Best guess as to what causes the bug is if you fail to reach it once the timer hits 29 seconds, it will explode if you try to dismantle it.
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Maybe you need to use your eyes. You can't turn off your brain and just run around. First LOOK you might notice 1 mine spawns it's an A after it's dismantled 2 mines spawn a B and oh would you look at that an A! Huh and weirdly the A mine is shooting out electricity and is broken. It also has different word when dismantling "Deactivating". Also when you step in a real mine it'll take about 10 seconds to turn off but the fakes only take about 5 seconds if it suddenly jumps up to like 20% the moment you get on it you know it's fake.