This isn't really a bug but I think it might be unintentional feature.
As long as you have one character surviving during the battle, you can quit the match and restart it with no penalty, its a big waste of time but it basically lets you go back and predict every move the AI will make, and easily outplay it.
I'm not saying multiple attempts per node for those hard fights is fun but it makes it easy to get every reward
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Yes, that's a standard feature, not a bug. Just tap the little gear in the upper left corner to activate the Retreat menu.
Hey, no shame, I had some guild members discover this feature recently. People who had more than a couple 7* characters, so it's an easy enough thing to overlook.