Good evening Stefano and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
I would like that the series of generated events have the same number of days as their father, but the series events last only one day.
This used to be a glitch with the Events Calendar Pro a few versions back, but the good news is that it is gone from the most recent version of our plugins.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
The second possibility is that there could be some view customizations in place in your WordPress theme that require updating.
A quick test would be to try temporarily reverting back to a default WordPress theme such as twenty-sixteen and see if that helps.
Let me know how that goes.
Best regards,
Geoff B.