Okay. You need to stop declaring the improbable the impossible.
The dice rolls are among the easiest things the studio can implement. Creating scenarios/exceptions where the player suddenly rolls lower... That is actually pretty hard. Considering that the studio have yet to properly balance out of the data they collect from us players, do you honestly think they'd implement such a scheme? To what end? To not make us like the game? It's part of the game, streaks happen.
Your calculation is off. Luckily, you provided an easy to calculate figure.
If the base chance is 50%, that means that 10 sides of a D20 result in succes (2 to 11). With advantage, that chance increases to 75%, because 50% twice. Note that, even with advantage, and a seamingly high statistic, you now have a one in four chance to fail. Yeah, that happens, even eight times in a row. It's hard to reproduce if you want it to happen, but it still happens. So does rolling Yahtzee on your first turn, but I've done it. Those odds are lower; 1 in 1296. I can assure you that I haven't rolled that many times, even though grandma really liked that game.
To quickly address your initial numbers: On an advantage roll where your chance is 85%, the target roll is 9, giving you 11 valid sides on a D20. You have a 60% chance per roll. To illustrate; you have a 40% chance to fail per roll, you have a 15% chance to fail at both rolls. Doesn't seem so weird now, does it? If it does, then... Well, I don't know what to tell you.
