Hi Jill,
Thanks for your message! I confirm the issue with Master/linked video objects. In V12 it is NOT possible to copy and paste a slide which contains a Master video / Linked video object.
If I start a new project in V11, create a series of slides with Master/video objects, these slides are being copied/pasted correctly.
Please wait when we fix this problem for V12. I'll try to do it quickly.