Wait, the ID of Swiftness is 1, 8290 looks like a potion damage value to me. Also, for Swiftness II you would want to set the fourth parameter (amplifier) to 1 (or 2, in the code it is 1, I have no idea for the command). Hmm..
Confirmed on 1.5.1. Very, very annoying and also makes the prespective look weird, especailly in the fourth screenshot (eating Rotten Flesh while looking at water).
I don't think this is a bug as carpets have no hitbox, meaning the game thinks the player is walking on the material below.
...but, it would be better if this is fixed as it would make the builds using carpets more realistic.