Export to CSV button (default/custom) not aligned properly

When I add an export to CSV button (either default or custom) the button is not aligned to the table properly. I would have expected it to be anchored left or right of the top of the table table but it is positioned about 35% from the left. I have tried just the basic default button, the custom button and fully custom button and all have the same issue. The code is as per the examples. Any idea what could be wrong? I have also tried different resolutions and they all have the same issue.