Hi @Yechiel Hadad ,
This behavior is by default where you cannot see values of toRecipient and ccRecipient but bccRecipient as null because you can't get the bcc recipients. It defeats the purpose of bcc/blind carbon copy and it aligns with RFC 2822 !!
As per the documentation, please use $select query parameter to get the values of toRecipient and ccRecipient
If you are still facing the issue I would recommend you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from
http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote. If you have any further questions about this answer, please click Comment.