The cost reduction IS staying. Improving it to 40% killing adds +25 rage cost to it. So that turns 20 into 45. It's not a problem or glitch, that's just how it works.
In my latest (mage) game, I haven't bothered improving soul drain any further than 30% for 20 rage because my max rage is only 50-something (56, IIRC), and it just isn't worth it.
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