https://arstechnica.com/gadgets/2017/09/apple-file-system-in-macos-high-sierra-wont-work-with-fusion-drives/

MacOS High Sierra will come out of beta and roll out to the public next week. If you have previously installed the beta version, you may need to take extra steps before installing the release so your Fusion Drive-toting machine doesn't experience any negative consequences. Apple announced that the new Apple File system (APFS) won't immediately support Fusion Drives and will only support systems with all-flash built-in storage in the initial release of High Sierra.