Inbound emails are not visible in the instance. Logs indicate the email is received by the instance but when you search that email with the sys_id in the sys_email table, it doesn't show anything.
Read ACL for sys_email fails - sys_security_acl.do?sys_id=8de12cdfc0a8016701fdf6b2bdb041cb
The admin override may not be working because of another property - glide.security.admin.override.accessterm set to false
Create or change this property "glide.security.admin.override.accessterm" and set it to 'true'.
If you don't want to set this property, the other option is to create the property 'glide.email.email_with_no_target_visible_to_all' and set it to 'true'.
1. "glide.security.admin.override.accessterm" - https://docs.servicenow.com/csh?topicname=t_EvalAdmOverrideAccLevel.html&version=latest
2. "glide.email.email_with_no_target_visible_to_all" - https://docs.servicenow.com/csh?topicname=r_AdditionalProperties.html&version=latest