Which monitor is the one that has the menu bar on it? You can find this by going into System Settings > Displays, then clicking on the Arrange button in the lower right.

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Whichever monitor has the menu bar is considered the "primary" monitor, as far as macOS is concerned. From my testing, there are some dialogs that always pop up on the primary monitor, which I've reported to the developers with bug id# 5070. As a workaround, try moving your menubar from the Samsung to one of the smaller monitors (click and drag the grey bar in the Arrange window in System Settings > Displays).


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