Why was the position closed at the price that was not on the chart?

On the chart you can see the Bid Price by default.

Long positions (Buy) open at the Ask price and close at the Bid price. Short positions (Sell) open at the Bid price and close at the Ask price.

To control the trading activities more attentively, clients can enable the "Show Ask line" parameter (chart - properties- common- show ask line or right click the particular chart - Properties - Common- and tick the show ask line)

After this command has been executed, an additional horizontal line corresponding with the Ask price of the new bar will appear in the chart.