[Senna]

Well, once you open preference in game, you'd scroll to the bottom and turn off Battle Background. Turning that off will remove it as well as the background visuals (Aside from the battle-map) but turning it on will only reactivate the problem. From my understanding, it seems the tool bar code that's suppose to let it vanish once entering the battlefield might be the cause whenever battle background is on however the code works perfectly fine whenever the battle background is off.

Image example:

With battle background off

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With battle background on