Hi Thierry,
Thank you for contacting Syncfusion support.
In the provided image, time duration for second appointment alone edited first. We suspect then detailed window is opened through Edit Series option (since the start and end time is 10.30 to 11.00) and clicking the Save button will display the alert where all the occurrence will be changed. It is default behavior based on google calendar.
Note: If detailed window is opened through Edit Event option, then the changes will reflect only on that appointment.
Regards,
Karthigeyan