This is because Basic Availability Groups do not support backing up databases on the secondary replica. When using VSS to backup a virtual machine that is hosting a Basic Availability Group, if the virtual machine is currently hosting databases that are in a secondary state, starting with SQL Server 2016 (13.x) SP2 CU2 and SQL Server 2017 (14.x) CU9 those databases will not be backed up with the virtual machine. For more information on the VSS, see your Windows documentation.
The VSS captures and copies stable images for backup on running systems, particularly servers, without unduly degrading the performance and stability of the services they provide. The VSS provides a consistent interface that allows coordination between user applications that update data on disk (writers) and those that back up applications (requestors). The VSS is a set of COM APIs that implements a framework to allow volume backups to be performed while applications on a system continue to write to the volumes. Use the SQL Writer Service to permit Windows backup programs to copy SQL Server data files while SQL Server is running. When the SQL Writer Service is not running, backup programs running in Windows do not have access to the data files, and backups must be performed using SQL Server backup. When running, Database Engine locks and has exclusive access to the data files. The SQL Writer Service installs on all operating systems. To configure the service, use the Microsoft Windows Services applet. It must be running when the Volume Shadow Copy Service (VSS) application requests a backup or restore. The SQL Writer Service is installed automatically. The SQL Writer Service provides added functionality for backup and restore of SQL Server through the Volume Shadow Copy Service framework.
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