OK, well I knew this was screwy before, but now I've tested it enough to know the pattern.
If I have objects set to customized y values like -4.5 and I copy the group, then cntl v to paste it on the map, the y value of the multi-select will show the correct value of -4.5, but when previewing the map, they have reverted to the 0 value up in the air.
I have to manually change the y value of a group to something else, before then re-entering the correct value of -4.5. Then they load properly on a map.
Basically I have to undo and re-enter the y value of a multiple selection.
To test, put a set of long candlesticks beneath the ground, but sticking out of the ground, set to absolute zero. -4.5 y may work depending on the map.
Copy a group by selecting the first and holding down cntl + alt while adding the other candlesticks. With the group selected, hit cntl c, then cntl v. Notice the y value of the group.
Load the map to see that they are now up in the air. Go back into the Foundry and change the y value of the group to 6, hit enter, now change it back to -4.5.
Now load the map to see that the y value of the group is now correct.