iPhone Calendar with Shared Outlook Calendar - not syncing from iPhone -> Outlook

I read already a lot about sync problems with iPhone / Outlook calendar - but not about this specific topic.

In general the iPhone is syncinc with Outlook.

But we are two person:

Person A created an Outlook Calendar and Shared this one with Person B with Edit Rights.

Person B can see in the iPhone calendar all entries from the shared calendar.

Here are situations:

Person A creates an entry in shared calendar - synchronized to Person B -> Works

Person B changes this entry - e.g. title - synchronized back to Person A -> Works

Person B creates an entry in shared calendar - this item is not synced back to Person A

Person B creates an entry in iPhone Outlook App - this item is synced back to Person A

So the 100$ question is: why is a new entry from Person B in shared calendar in iPhone calendar app not synced back to Person A??? It worked about two years ago, but since then I don't know what changed and never had energy so solve it - but now I want to know what's the problem.

Any idea is highly welcome!