Group 600 is the masters of five multi-cell intensity only fixtures.
Group 601 is the cells of those fixtures.
[Group] [600] [+] [Group] [601] [Full] [Full] sets only the master intensities to full. It leaves the cell intensities where they were, even though the cell intensities become selected.
[Group] [600] [Full] [Full] [Group] [601] [Full] [Full] works as expected.
Eos 2.9.1