Hello @Hristova, Dayana
*Since this is a user-to-user community forum, I can only provide guidance and troubleshooting suggestions based on my experience as a user and the information you have shared. *
Based on the information you provided, I tried to reproduce the issue by following the same steps you described and sending an email with a Word document attached.

Unfortunately, I was able to reproduce the same behavior. The email was not received by the recipient, and I could not find it in the Sent Items folder or elsewhere in the mailbox.
Therefore, I would recommend contacting Microsoft Support for further investigation. The support team can work with you to collect the necessary diagnostic information and investigate the issue further. You can follow the guidance here: Contact support and provide feedback in new Outlook for Windows. Additionally, you can also contact your IT administrator and ask them to raise a support ticket through the Microsoft 365 admin center on your behalf by following Get support - Microsoft 365 admin | Microsoft Learn.
In the meantime, please continue using the available workaround to send the attachment while the issue is being investigated.
I hope this information is helpful. If you have any further concern please feel free to share with me any time.