High Sierra install fails with 'invalid request'

I've had several iMacs running fusion drives and had installed the High Sierra beta version 10.13 that converted the fusion drives to APFS. They all ran without issue. Now that we are several updates into High Sierra, I had a few programs that wouldn't update because of an old version of the OS. So, I decided to bite the bullet and backup my data via Time Machine and revert the fusion drives back to JHFS+ and reinstall High Sierra 10.13.4 (without APFS). All is fine on every Mac except for one. The iMac in question is a 2015 model that has a 4TB hard drive and a 128GB SSD (Apple blade). When I followed the instruction provided by Apple to convert the fusion drive back to JHFS+ and reinstall High Sierra, I get about a quarter way into the OS install and a windows pops up saying High Sierra can't be installed 'invalid request'. I've tried breaking the fusion drive, re-combining the drives, erasing the drives, running first aid on the drives, doing every trick I know and still get the 'invalid request' when trying to install High Sierra on the fusion drive. I can install Sierra 10.12.6 on the fusion drive, but then can't transfer the Time Machine backup because the OS used for Time Machine is newer than the OS on the computer. I can install High sierra on the 4TB drive by itself without fusing it to the SSD. But, that's not the way I want to go. Anyone have an idea what may be causing the 'invalid request' when I try to install High Sierra on this 4.12TB fusion drive formatted in JHFS+?? It worked fine in High Sierra bate in APFS as a 4.12TB fusion drive. I'm stumped.