FYI/PSA to others. During the Spoiler:
I killed the first boss with god mode on with no problem in my demo video.
God mode only seems to affect 2 specific boss fights. The one listed above and the Lord of Hollows, who heals himself to full on every hit of his own minions.
It could purely be a bug in my save though and I finished the game already on that save so I only wanted to post it up as a warning in case others had the same bug. Starting an Apoc run this weekend so I'll know if it comes up again and will post my savegame/location if required then.