This is currently by design. There was an issue back in the AppAssure for Linux 5.3.1 builds where there was found to be corruption in implementing incremental snapshots after reboot.
Since 5.3.2 this feature has been taken away and so if the agent reboots, there will be a base image taken on the next snapshot.
Development has been working hard to bring back this feature of incremental backups after reboots as they know it is beneficial for the customer.
However development wants to make sure they get the process right so that in bring back this feature they do not allow corruption back into the Linux for AppAssure product.
In a future release of AppAssure this issue will be resolved.