I've never seen that happen. Only time I have noticed my cost basis changing is due to wash sale, purchase or sale, or a split.
If you ran the tax optimizer it may have adjusted your cost basis if possible. For example, say I bought 50 shares of XYZ at $50 and another 50 shares of XYZ at $100. My cost basis is $75. I then sell 50 shares. Your cost basis can change depending on what 50 shares you decided to sell. Did you sell the ones purchased at $50 or the ones purchased at $100? If you elected to sell the shares purchased at $50, then your cost basis is now $100. If you elected to sell the shares purchased at $100, then your cost basis is now $50. By default, I think your cost basis would be $75 unless your broker tries to minimize your realized gains by default or it may try to realize long-term gains over short-term gains where possible for tax efficiency.