When switching a track from mono to stereo, S1 will randomly pick a stereo input as the new input for the track. Instead it is preferable to keep the input as the existing mono.
Let's assume that when a user switches from mono to stereo, there is a 50/50 chance that they also want to change their input to stereo. IMO it's actually more likely that they will want to keep it mono, but for sake of argument let's say 50/50. Now in the event the user DOES want to use a stereo input, there is still the question of WHICH stereo input they want to use. Personally I have 8 stereo inputs at my studio. S1 randomly chooses a stereo input, so there is a 1 in 8 chance that it picks the input I actually want.
Now let's compare possible strategies to choosing a track input when user switches a track from mono to stereo.
Strategy 1 (Current Approach): Choose a random stereo input. Probability of picking the right input is 50% * 12.5% = 6.25%
Strategy 2: Keep the mono input. Probability of picking the right input is 50%
Clearly, strategy 2 has a better chance of picking the correct input.