MT is always showing the latest value coming from the datafeed. RTH for hist Backfill request. It has no impact on the chart showing data that is already in MT, and the last bar on hist chart comes from Open, High, Low, and LAST on the quotes. There is no way for us to reliably know what the official close is.
Though you could mess with switching sources, that will not reliably get you the RTH quote. if you want RTH on historical, turn off the real-time option for the historical charts.