Yup, I have seen the same thing. Posts that were going to pay me around $50, suddenly dropped to $25. How demoralizing.
If you buy a sandwich, eat and like it, the person who make that sandwich shouldn't have money taken from them days later, just because you decided to buy a sandwich at another store.
Once you make a purchase (up-vote), you should "own it" and the amount paid out to the author should never go down.