This is a known problem. I suggested a fix in
this thread, if you agree, please send Omni some official feedback.
For now, what I do is just set the start and due dates for the group as a whole, rather than for individual actions. This has the bad side effect that you now have to go back into planning mode to mark the entire group as completed before the next round of actions will show up. But it's the only workaround I know of.