From the release notes to macOS High Sierra beta 2 update 1:
What does this mean? Is it not safe to back up using Time Machine starting with 10.13b2 Update 1?
As it plainly states, it is absolutely impossible to back up a beta 2 update 1 root volume, and previous beta 2 advised disabling Time Machine.
You didn't install this to your primary machine and upgrade your primary stable OS installation, did you?