im not using any texture packs or addons. it only happens when vsync is off and fps is set to unlimited and entering a world with vibrant visuals is on. its unlikely to happen if entering a wrold with vibrant visuals off and than turn is on in game. but when it happens it is still there after tunring vibrant visuals off in game. the textures it effects change everytime re-entering a world. and when language is set to simplified chinese(and maybe some other languages) it even effects the text in some places like settings.
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This issue is being tracked at MCPE-202009.