Many traders sell their holdings during the day and use those funds to do other intraday trades, and then buy back the stock before the end of the day. They don’t want their holdings to change because capital gain taxes are lower for investments.
Showing such CNC sell trades in positions makes it easier for traders. They can also see the MTM profit/losses on this together with all their other intraday trades.
If you don’t intend to buy back the stock before the end of the day after selling, disregard the changing P&L for the CNC/stock sold showing up in positions page. It doesn’t make any difference to you.