When moving a hostgroup between environments all previously associated puppet classes are lost.

Example:

ensure "myclass" is available in both environment A and B
Create hostgroup "hg1" in environment "A" and associated "myclass" with it
move "hg1" to environment "B"

Expected: hg1 should still have "myclass" associated with it
Actual: hg1 has no classes associated with it

The same occurs when cloning a HG from one environment into another.