Apple Advises High Sierra Beta Testers That APFS Won’t Be Supported for Fusion Drives at Launch ⇥ beta.apple.com

Apple:

Beta versions of macOS High Sierra made a change in the disk format of systems by converting them to use the new Apple File System. The initial release of macOS High Sierra will provide support for the new Apple File System as the default boot filesystem on Mac systems with all-Flash built-in storage. If you installed a beta version of macOS High Sierra, the Fusion Drive in your Mac may have been converted to Apple File System. Because this configuration is not supported in the initial release of macOS High Sierra, we recommend that you follow the steps below to revert back to the previous disk format.

If you’re part of the AppleSeed beta program or are an Apple Developer using a Mac with a Fusion Drive, you’ll have to back up and reformat your drive to HFS+ if you’re upgrading from a beta copy of High Sierra to the GM.

Based on the way Apple is framing this notice, it sounds like the APFS upgrade was a bug or, at least, unintentional. After all, APFS is designed to be used for a primarily solid-state storage world, not necessarily spinning hard drives.