I've never use Office for Mac, so for what it's worth:

If you have both tabs selected—ie highlighted—when copying, try selecting only the second sheet.

If you use a keyboard shortcut to copy, could it also be assigned to deleting? How exactly are you activating the Copy function?To be clear, when you copy the chart in Sheet2, it deletes Sheet1—is that what you're saying?

If you make a copy of Sheet1 first, does the copy delete both Sheet1s?

This is bizarre behavior, and would have plenty of commentary on the web if widespread. So it must be something odd in your setup. Is their a Repair Office command you can run?