I confirm your problem, for 6 months I had a purchase price that did not move, and even without buying back and without selling I saw my purchase price of several cryptos increase
Mango_Fortune
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Another thing I noted on binance is that if price of the coin is for example 0.72$ and you buy coins and see the average price of each coin bought will be 0.74 or 0.75 while price didn't fluctuate. Is it a fee or what I don't know. If anyone has knowledge please let me know .