Please see the attached prints.
The first group has 16 events. And after expading it only shows 14.
It only shows 16 after expading the second(bigger) group.
Could you please post here the event data that you used reproduce the problem?
I created a pastebin with it. https://pastebin.pl/view/1e00568b
You will notice that the last event of the second group (right), will only show if the first one is open.
However, it looks like I'm not able to reproduce the problem using this sample data set. Are you using the latest DayPilot Pro version? If it still doesn't work, could you please post the complete config?
Sorry - I can see the problem now. I'll let you know as soon as I know more.
This problem should be fixed now in the latest sandbox build (2022.1.5239). Please let me know if there is any problem.