We have experienced the following in Sugar 4.20d which appears to users as being lost Sugar data.

Create a call to an account.
Oops - meant to link it to a contact - so edit the call and on the right hand side, select Contacts and select a contact.

The call does not appear in the activity list of the contact, nor is it searchable in the call list (although it is there if you can find it).

Workaround - make sure you select the contact as an invitee and it appears in the Activity list for the contact.

Curious - when you schedule a call directly for a contact, it appears in their activity list even though they are not invited to the call.

I have not yet worked out the link between contact and call and invitee list, where the contact can have the call in their activity but not as an invitee. I would have thought this was calls_contacts.