The Portfolio Manager only tracks the daily share prices.
If you sell, buy, or your share is delisted, you'll need to manually update that position. It won't automatically update this for you.
Go to your current holdings and find the stock.
Click the cog/gear at the left then choose Record a Sale of ...
At the top, the Record a Sale box appears.
Enter the quantity sold.
Enter the sale date, the sale price, and any brokerage.
You may need to consult your share broker or other documentation.
Click 'Record Sales Details'