There are some limit situations in which the scheduler stacks appointment on each other due to bad layout handling. It seems that the order and the belonging calendar is important in order to reproduce the issue.
The problem can be reproduced using the SQL inserts (see attachment) properly (calendar ids needs to be aligned to current data configuration). If "TEST 1111" event is moved ahead at 10:00 the issue is clearly visible: "TEST 2222" is lost under "con luca" and then "danzo" appears.
This issue was introduced resolving
WTCALENDAR-53 in which event ordering is changed from date-time to calendar/date-time.