Why animation in Anim Bled at half speed is played super slow?

[Remblej]

I have simple Animation Blend for 8-dir locomotion like this:

My character’s movement speed is dynamic (can be changed by buffs and debuffs). My expectation was that if my character is running at half speed (250), animation would match it, but it doesn’t.
It’s actually visible even in preview - when I put marker (with Ctrl) on 500 speed, animation plays nicely, but when I put market at 250 speed, animation is super slow-mo instead of half speed.
Like so:

Is this expected and I’m completely missing something? Or is something miss-configured?