Oh sorry. Sub-group = party. Raids are comprised of up to 8 parties or sub-groups. :)
Anyhow, what happened was if I had it set to 2 parties per column (10 units per column), then the members of those parties would be jumbled up. Instead of just drawing 2 parties next to each other, so I could still correctly see who was in which party, it would be as if there was now a 10 person party.
For example: say I had a 10-man raid aaaaa bbbbb. With GridDynamicLayout set to 5 people per column, this would be perfect. But at 10 people per column, I would see something like aababbabab.
Since not everything is raid-wide yet, it's still sort of important to know exactly who is in which party.
Something I noticed back when I still used this with Grid (and nothing in your changelog says this might have changed) was that when you had more than a single sub-group in a particular column (or row), the members of those sub-groups would be jumbled up.
It seems sub-group membership was being ignored in preference for honoring column (or row) membership.