No velocity with slime

What exactly is the bug here?

Soul Speed doesn't increase your speed on any other block that slows you down, like Ice types and slime blocks. Only soul sand and soil.

I believe you're trying to say that armor stands (and players?) move the same distance on soul sand for a given initial velocity (passed on by slime blocks) no matter the soul speed enchantment level, while it still differs from armor stands without enchantments. That's at least what the pictures convey.

But if you were to compare it with the same test on soul soil, you'd notice that all the enchantment does on the armor stands is take away the penalties of stronger deceleration(and slower running speed) of soul sand. On a player, soul speed also affects how fast he can run. Armor stands do not run. They have their speed set by the slime block.

So while it looks weird, I think this is working as intended.

I'm not entirely sure if this was the issue. A more elaborate description would certainly help.