Its not just that, but why test quest failure for if the companion is dead, if in principle companions can't die? This is a typical case of game design where lower level engine systems like physics dictate behavior/cause bugs in higher level functionality like the quest system.

Game engines do everything every frame by default, and the game has to disable/enable stuff from happening, which leads to bugs like this. With a more functional approach instead of tweaking the engine state these bugs can be avoided.