Outlook graphQL Api reading contacts misses a lot of contacts

I developed a query for Outlook contacts using GraphQL via the API.
However, in the Outlook web interface, you can see that under ‘All Contacts’ there are many more contacts displayed than those I get through the API query.

The graphQL query considers Default, ContactFolder, ChildFolders, and User. Despite this, the API query only returns about 90 contacts, while under ‘All Contacts’ in Outlook, you can see that there are about 1000 contacts available.

The Outlook account is running on an Exchange server. When looking at the details of the contacts, it is noticeable that the only difference is that contacts with the identifier V16 are not synchronized, while those with the identifier V12 seem to always be synchronized. It is also not clear whether the contacts that cannot be queried via the API actually come from another source, e.g. from another server.

The question: How can you find out what the issue is?
Could it be a permissions problem with the Exchange server, so that the API cannot query all contacts or directories? There are no error messages. Apparently, the GraphQL query actually only finds about 90 contacts.