This is in latest version as of today's date. An annoying bug which was not present in older versions of illustrators and is reproducible:

Create two vector objects and group them
Duplicate this group (this is just to provide two groups, the problem exists across all groups of objects)
Use Direct Select (A) to select one object from within each of these groups
Copy (Ctrl+C)
Paste in Front or Paste Behind will paste one object in front or behind and move the other selected object to that depth
Regular paste pastes two objects as expected.