This is a very minor issue, hard to find and easy to get around. Still, I don't think it's intended to happen so I might as well post it, it could be part of a greater issue for all I know.

(recording discarded some of the frames this happened on, it happens every time the animation was disabled)

The size of the bone seems to reset for a single frame after deactivating an additive animation, (at any alpha, "applied as normal instead of being mixed out" or not).
The scale used in the additive animation doesn't seem to matter either.

To reproduce:
-Use any visible bone on Spineboy (front-upper-arm in my case)
-Make a new animation (SIZE_X), separate the bone's scale, set Scale X to 1.1
-Make a new animation (SIZE_SAME), key Scale, leave it at 1.0 - 1.0

-On playback, set SIZE_X first, as additive or non additive, any settings
-A slot after, set SIZE_SAME with mix 0, as additive
-Enable and disable SIZE_SAME
SIZE_X's scale is reset for a frame every time SIZE_SAME is turned off

This occurs with all separate-able transforms, not just scale