If you notice that the things you are debugging behave strangely, values are assigned and the debugger won't tell you where or how, or actions are done and you have debugged the whole thing without a trace of how the action was done, this is one to check out. The system is delivered in many components, and SAP has helped us developers to avoid having to plough through these standard components. So in effect, your debugging session should not be too bothered or influenced with coding that is exected as part of "the system".
Of course this does not mean you can't debug it. SAP is an open source system (as far as Abap coding goes) and thus there is "System debugging". Switch it on via /HS or via the menu: System > Utilities > Debug system. Switch it off again in the debugger itself under Settings > System debugger on/off. You'll find the debugger takes you to a lot more detailed coding. You'll also find that the problem you are looking for is not likely to be in there...