1. It's a beta - an early beta. Why would you need to have the system provide full backups of a version that may have issues, and where Apple may, or may not, provide full information about those issues?
2. It's a beta. Report your issues, and continue to
NOT use a beta system, which may or may not be completely stable, for your primary computer.
Yes, everyone on High Sierra is in a (potentially) risky situation. (Not to repeat myself (again), but it is a test system until the final release
)
If Time Machine is presently disabled, and you feel the need to keep a full backup, then you will need to use an alternative.