To reproduce: Make a long minecart track with powered rails at the beginning, then ride the minecart down the track. The minecarts practically don't slow down at all.
In the screenshot: An empty minecart stopped at the place where the glowstone and minecart are in the picture, while a minecart that the player (or a mob) is riding made it all the way to the end at much the same speed.
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Yes, since they practically don't reduce speed at all on regular rails (I added a screenshot explaining the problem).
Are you sure that that's just a bug? Do they slow down less in this snapshot than in other ones? If not, then it was probably intended, as occupied minecarts have different physics than unoccupied ones.