This is a built-in feature that shows an event lasting longer than 10 hours as a full-day event.
You can disable this function with the following steps:
1. Click on Advanced in the Preferences menu
2. Under the PERFORMANCE header, you will find 'HW accel. on week view'.
3. Untick the box next to HW accel. on week view.
When this box in not ticked, then your 10-hour event will not show up as full-day anymore.