The Blend Space's Per Bone Blend doesn't always apply its interpolation speed correctly to the appropriate bone or at all.
Open the content examples project.
Open the owen locomotion blend space(press spacebar to pause the preview).
Under 'Sample Interpolation', set 'Target Weight Interpolation Speed Per Sec' to 0.01 and add a Per Bone Blend with Bone Reference of index_03_r and Interpolation Speed Per Sec of 100.
Adjust the blend space preview value(green marker) back and forth.
Result: The left arm moves and it shouldn't.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-79649 in the post.