Thank you for your post.
I am unable to replicate the issue using FileMaker Pro Advanced 16 under Windows 7. Each time I grab another script step, that script step is automatically highlighted as the previous script step is deselected, so when I move the new script step above the previously selected step, it moves correctly. I perform a "grab and slide" in one motion.
Do you have a specific OnTimer script installed when this occurs as this might interrupt the mouse clicking and timing? That is, if the clicking and dragging is being trapped, the clicking may select another script step as you begin the dragging.
Any plug-ins installed? If so, try disabling them and see if the issue continues.
Any other information you can provide may help me replicate the issue.