It depends on which version of the beta was the first you installed. Early betas converted Fusion Drives to APFS, but later betas did not.
You can pretty well assume that your Fusion was still on HFS+ - never converted to APFS. If it was on APFS, the GM installer would have prevented the install, and given you a link to the procedure for converting back to HFS+.
Paging captain obvious. LOL. Of course, if your drive says it's HFS and you're on High Sierra, you didn't install an affected build. If you waited for GM, then there is no issue here.