This is probably just the data that Minecraft gives, thanks Mojank. But it would be helpful and more intuitive if there were a special case for Pace Purification for arrows stuck in blocks to have zero velocity.

No, that's expected behavior...

Note: this apparently can be worked around (by raycasting from the arrow's position using its velocity and seeing if the resulting position is within a block) so it may or may not be necessary to patch this.