Hi,
I’ve shared a calendar of a folder with 15 subfolders, the toggle SUBFOLDER ITEMS is enabled, I can see the tasks in side Infinity but they are not displayed in MS Outlook.
I made a test sharing the calendar of a subfolder and it works.
Looking into the documentation I didn’t find any reference the management of the toggle SUBFOLDER ITEMS is not included in iCalendar so, I suppose it is a bug.
Thanks
Hi @luca.colombi welcome to the community, we are happy to see you here 
Regarding the bug report you’ve created, unfortunately our iCalendar feature works per folder basis, so - if your main folder has sub folders, subfolder items won’t sync all together, regardless if you have toggled on " show subfolder items" in infinity.
In this case, you will have to create a calendar tab in each individual folder and sub-folder and then use those iCalendar URLs and connect them with your google calendar, outlook (or any other) individually.
We will be working on improving this in the future so that it does not force you to create dozens of calendar tabs and links in order to get all the items from all the folders and subfolders. 
I hope that explains it all 