SCS - Is it normal to experience mild GI discomfort when taking probiotics?

Yes, even if you have had previous probiotic experience or have been on a probiotic for a while, introducing different strains may potentially cause GI upset to occur since a different part of the GI tract is being balanced/restored.

But many people do not experience any discomfort!

This is completely normal, and in most cases only persists until your GI fully adapts. Once fully adapted many people experience no further symptoms.