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Is it possible that you did not set your team to go for baron? If the enemy team always takes it uncontested that would be a way for them to close out.
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it happens all the time https://ibb.co/f2Hzq5 30 something to 0

my friend even lost a game that was like 10 min hitting their nexus total win and lost cuz of enemy minions he posted on the forums too.