We have examined the above mentioned issue with displays and ARCHICAD palettes. This behaviour is caused by a bug that is planned to be fixed in the next regular update of ARCHICAD 18.
The problem is caused by the fact that since OS X 10.10.3 the operating system sends updated info about the movement of the mouse less frequently to ARCHICAD. So thus ARCHICAD "is not informed" that the palettes were dragged to the other display, their old positons are stored. When you switch between applications, ARCHICAD will show the palettes on their stored locations.