Update Aug 12 2020: Apple has released a supplemental update for macOS Catalina 10.15.6 that reportedly fixes the virtualization bug. The supplemental update is available now via System Preferences -> Software Update.
In the latest version of macOS Catalina 10.15.6, a bug was introduced that can cause random reboots when running virtualization software such as VMware Fusion and Oracle VirtualBox. Unfortunately, at the time of writing there are no workarounds other than reinstalling an earlier version of macOS Catalina (10.15.5 or lower), or shutting down virtual machines when they’re not in use and rebooting the host Mac regularly.
This bug appears to affect macOS users running various virtualization platforms on all Mac models that support macOS Catalina based on anecdotal evidence.
At this time, we recommend users running virtualization on macOS Catalina hosts hold off on upgrading to macOS version 10.15.6.
Customers experiencing this issue should open a support ticket so that we can assist further.