Yes, you’re right that the start events aren’t coming in because their start dates and times
aren’t exactly equal to the parent event’s start date and time.
Try using 2 criteria for your sub-events in your nested RG:
start >= Current cell’s grouping start change hours to 0 change minutes to 0 change seconds to 0
start < Current cell’s grouping start + 1 change hours to 0 change minutes to 0 change seconds to 0
Also, I see that ignore empty constraints isn’t checked. This might not be impacting you on this search, but it’s usually important to have this checked.