The software simulation will perform a bit differently for some functions, but what you have experienced is correct.
In simulation mode (no connection to hardware), you are emulating a hardware sensor by clicking the inputs in the simulation window. Therefore, the input indicator will stay lit while you are holding the left mouse button or once you have done a right click as if a sensor or button were connected to the input and activated.
In hardware monitoring mode (online monitoring), you are linked to the actual PLC I/O and data. The logic state of any physical inputs will primarily be controlled by the hardware connected to them. When you click on an input from online monitoring, it will activate for one program scan and will be utilized by the program for that scan, but when the PLC does a hardware input scan, the input status will revert to its current state. If nothing is connected to the physical input, then its default state is inactive (logic 0). This is why you only see a brief flash of the input indicator in online monitoring after clicking on the input, which is essentially a one-shot as you mentioned.