Hello, I don't know how the virtual oscilloscope in Multisim is implemented but I would consider two things:
1. The 1ps rise and fall times are very small compared to your period and are not realistic. I would set those to some more sensible value. For a 10s period, rise and fall times in the us or even ms range are plenty to obtain a square wave.
2. In a real oscilloscope the sample buffer length is finite and above a certain time base setting the oscilloscope will switch to continuous acquisition. It means that you will see a scanning/running waveform instead of a steady curve. This is also related to the sample rate. Again, it depends how much the virtual instruments mimics the real one.