From the release notes to macOS High Sierra beta 2 update 1:
"New Issues
Time Machine is disabled for macOS High Sierra 10.13 beta 2 update 1 (17A291m). Apple recommends turning off Time Machine for APFS volumes on earlier versions of macOS 10.13 beta 2 as the space for snapshot files may not be freed correctly. (33006905)"
What does this mean? Is it not safe to back up using Time Machine starting with 10.13b2 Update 1?