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This trading down occurs at intervals, if there are huge natural trades going on, you won't notice this and the exchange will be silently making money, but the price will be going up slowly or be about stagnant. This can be verified when you analyze exchanges with big real volumes like Binance and big wash trading volumes like Bitforex or bibox. Also, most times it is visible that wash trades are going on because you'll see orders which are short lived say few seconds on order book. Now when this goes on, your portfolio price doesn't naturally go down..it is manipulated down..If these exchanges trade the prices up, they would love money.
This is common practice for many exchanges, including stock exchanges. There are market makers whose responsibilities include creating liquidity and reducing volatility when possible. You may perceive this as unfair trading. Of course, MMs take every opportunity to make money. We know that most crypto assets are so illiquid that even $100 can shift the market. And creating liquidity can be beneficial to customers.