stock sale capital gain calculation on 1099 form brockage firm

Hi, 

I would appreciate some help.

 

I bought X shares of XYZ on 01/05/2024 and sold X shares of XYA a few days later with a little gain. During  this time, I still have large amount of XYZ shares in my portfolio which I help for many years.

 

On 1099 from the brockage firm, when calculating capital gain, it picked X shares from years ago which had a much lower price, this resulted in large capital gain, though LT gains. 

 

What I meant is to use the buy/sell pair within the days in Jan/2024 to calculate my gain.

 

Can brockage firm correct this after the order is completed and settled?  If not, I can change cost base myself when filing tax? (even though the cost base is reported to IRS on 1099 from the firm?)

 

Thanks

Dav