Summoned arrows aren't affected by a pushed upward slime block.

They don't, they just become invisible.
I think you will see the same effect when summoning a flying arrow.

Cannot confirm

Did you *(1)* Activate command block with "/summon Arrow [(X) (Y) (Z)" (The place where the slime block will be, or over it)] *(2)* Activate a piston under the slime block. Then watch the arrow disappear as the slime block is pushed up.

Closing as cannot reproduce, since this wasn't confirmed to be a real issue.
It doesn't happen in 14w21b.