The issue occurs because the scheduled scans do not use a logged on user account.
Mapped drives are not made available for on-demand scan tasks because the authentication for the scheduled on-demand scan uses impersonation. Impersonation does not have access to mapped drives. Drive mappings are determined at system logon, and security settings are established then.
This functionality is not expected to change, unless there is a significant change in the architecture of the Windows operating system. An account with sufficient permissions to access the network drive must be specified in the properties for the scan task.
