As written on MacRumors and other places Macs using 3TB Fusion Drives for the macOS and Boot Camp installations can't be upgraded to Mojave currently without losing the Boot Camp partition.
To fix this problem, Apple says that customers with the 2012 27-inch iMac with 3TB hard drive will need to completely remove the Boot Camp partition using Boot Camp Assistant before macOS Mojave can be installed.
After upgrading to macOS Mojave, Boot Camp will not be able to be used to install Windows on these machines. No other iMac models appear to be affected by this issue.
What is the technical reason for this?
PS: See also Apple knowledgebase document (which mentions only Late-2012 27-inch iMacs with 3TB fusion drives.)