I cannot explain why your broker would be adjusting the cost basis of the underlying stock after a covered call expired worthless. The tax treatment of an expired call is very straight forward. According to IRS Publication 550 (2024), Investment Income and Expenses (Including Capital Gains and Losses).
Writer of option.
If you write (grant) an option, how you report your gain or loss depends on whether it was exercised.
If you are not in the business of writing options and an option you write on stocks, securities, commodities, or commodity futures is not exercised (or repurchased), the amount you receive is a short-term capital gain.
If it is a short-term capital gain, it cannot be used as an adjustment to the basis of the stock.
You might want to call your broker for an explanation.
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