Following your instructions never takes me into the campaign screen. Following your instructions but entering the campaign screen causes me to leave the battle when I enter the screen. What helped me reproduce the bug is the screenshot that shows that you have a battle tab while in the campaign screen.
Here are the steps I found.
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Multiplayer -> Battle List -> Enter any battle.
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Back -> Singleplayer/Coop.
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Click on the battle tab at the top to minimise the battle.
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Click Campaign.
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Click on the battle tab at the top to maximise the battle.
What happens is that entering campaign leaves the battle, but it does not remove the top tab if the battle is removed. Perhaps the battle tab is not removed if you are kicked as well.
You can be in a battle and the campaign screen if you Join A Battle on someone in chat from the campaign screen. I couldn't cause any bug with this so it seems fine.
