I think this could be to do with the OnPreviewMouseButton overriding the On Clicked
I’m not sure you can directly fix this it might just be how the thing works, and i’m sure the standard OnMouseButtonDown doesn’t work with the drag system as it cant detect left click.
You could try handling your on clicked logic inside the OnPreviewMouseButton
Or
use a event dispatcher, call a function or call an event if you don’t want to handle your logic inside the OnPreviewMouseButton