Blending two clips together : illogical outcome ?

Hi forum, what’s happening is beyond my hability to understand so I think I need help…

I’m trying to add 2 animations of the same object (for example, an idle stance with miscellaneous hands or head movements). Everytime I switch the blending mode to “add”, it seems to divide the rotation value by 2. So I created a very simple example with just one rotating cube to understand.

Track A (replace mode) : rotation 45°
Track B (replace mode) : rotation 90°

Now for the tests :

Track A alone (add mode), tack B mute : rotation 22,5°
Track B alone (add mode), track A mute : rotation 45°
Track A (add mode) plus track B (replace mode) : rotation 67,5°
Track A (add mode) plus track B (add mode) : rotation 45°

For the life of me I can’t understand what’s happening. Is it too much to ask to have rotation A + B = 135°, like rotation A + rotation B ? How to do this ?

So, did I actually find a known bug ? It seems to have something to do with faulty quaternion addition :

https://developer.blender.org/D1929

All right, I’m now certain that it’s a bug due to a mismanagement of quaternions in Blender (v 2.78c), since the same settings with Euler coordinates produce the correct result.

test combinaison linéaire animation (quaternion).blend (437 KB)
test combinaison linéaire animation (ZYX Euler).blend (442 KB)