Hi Cinvestav,
To troubleshoot the issue, I would like to confirm the following things:
1. Do you mean the synced user account is deleted in Office 365, and the user exists in AD?
2. How long does the issue occur? Does the user account exist in deleted users from users and groups in Office 365 admin center?
3. If the user account is not deleted from AD, after running Dirsync, will the user account come back in Office 365?
4. Have you configured filtering for Dirsync?
Also, could you provide the detailed environment such as hybrid deployment?
Thanks,
Johnny Zhang