I have the same problem. No matter how I tweak Max Speed and default speed only runs the running animation. Even the values are the same with the blendspace speed and max walk speed. Only when I delete Run anim it plays walk. But this is no fixing of this problem. I am starting to go nuts. Sorry dude that I don’t have solution. I hope somebody help us 
I am thinking that may be it is problem because I a have custom character and every ■■■■ tutorial about this works with the UE mannequin. Nobody show this with custom. I think too that the max walk speed in the character has nothing to do with the variables speed of the Blendspace
If I find a solution I will post here.