Because this exposure fusion doesn’t happen in linear space at all, but after the base curve is applied, which is not linear and not even pure-power, and I tried to show you the code to justify my claim, but you still don’t understand. So it fails precisely because it does not happen in linear space. And you got your assumptions wrong because, as most opensource hackers, you treat RGB codevalues as random numbers and don’t look at the whole pipeline that goes from scene-linear light ratios to whatever broken transfer function you are trying to hack.
I mean, if you want to invalidate Parseval’s theorem, do it. You might even get a Fields medal from it. Until you do, I’m right, you are wrong. You don’t blend, blur, convolve, feather, or merge anything outside of a linear space, where RGB codevalues are proportional to light energy. Full stop.
There is absolutely no relationship preserved between R, G and B at the output of the base curve, because it’s not linear, and it happens before the exposure fusion. Once again.