Hmm. Strange. Have you tried different window modes, like Borderless Windowed, and normal Fullscreen? Borderless Windows are usually good for dual-monitor setups.
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There should be a tick box in the visuals somewhere to switch it from normal fullscreen to borderless. Now that you mention it, there may not be a windowed mode, and I remembered that wrong. I apologize for that. If you already tried the tick box, though, I would say make sure your resolution is set correctly. Or -- in games that have done this to me before -- try resetting from fullscreen to borderless, or the other way around and see if that improves at all.