You need to understand that AmIBroker is multithreaded application.
Everything runs in parallel and has "separate life". Debugger is NOT connected to any of your (possibly multiple) charts. It is separate, as every chart is separate from each other and separate from analysis window as well. These all run in parallel threads, living in their own worlds.
Debugger runs in separate (backtest) context, not in chart context, therefore it does not know/care what you clicked on of possibly many opened charts. The selected bar in the debugger is the last bar. Selected bar is individual/independent property of single chart.
The only global selection that works across all charts, analysis and debugger are global range markers accessible via BeginValue/EndValue functions, see range markers in the manual
http://www.amibroker.com/guide/h_charting.html