Transform (Modify) Bone 'Add to Existing' Scale in Component Space doesn't seem to work as expected

I’m trying to stretch a bone along its Y-axis (in component space) by feeding a vector from my character’s blueprint to the animBP.
If I start with a vector of 0,0,0 it doesn’t seem to add it to existing at all. Instead I get a crumpled mesh and errors:

"Warning Scale ‘‘X=0.025 Y=0.025 Z=0.000’’ is not valid on object '/Game/Levels/UEDPIE_0_Level1.Level1:PersistentLevel.Breathe3BP_C_1…

Obviously adding nothing to it isn’t what it’s doing. It appears to be calculating the difference and adding that instead.
Is that right? I’m just trying to understand.

I’m on v4.10.1-2791327 FWIW