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

To answer this question, it is important to understand that in the terminal charts are drawn in accordance with the Bid prices.

The Ask line may also be drawn on the chart (menu "Charts — Settings"); however, by default, it is simply switched off.

Thus, on the chart, you cannot see the closing prices of your short positions (Sell), as this type of positions is closed at the Ask prices (while the Buy positions are closed at the Bid prices).