The behaviour is very similar to what happens when all AMD KEXT’s are removed. My experiments ended up with only removing some AMD KEXT’s (https://forums.macrumors.com/threads/force-2011-macbook-pro-8-2-with-failed-amd-gpu-to-always-use-intel-integrated-gpu-efi-variable-fix.2037591/page-41 – starting at #1013). This tells me that some display functions are maintained in the AMD KEXT’s no matter which graphic card is active. Perhaps El Capitan is depending more on model information rather than detecting everything, and this has changed in High Sierra.
Brian