If you are adding those things it sounds like you must be doing an all grain mash. If you mash with *only* corn or rice, then there are not enough enzymes to break up the starches. If you are using a bunch of barley and adding some corn, then the enzymes in the barley will break up whatever starch they encounter, regardless of the source. You can control what reactions happen by changing the ratio of different grains and the temperatures and times they are exposed to in the mash.
When you make beer by adding a commercial yeast pack, you are adding like a trillion cells that are all from a single strain, propagated in a lab in sterile conditions. If that strain can't produce enzymes that break up complex polysaccharides, then those molecules will remain in the finished beer and affect the properties of the beer. That may be intended: undigested sugars and starches add sweetness and "maltiness" and mouthfeel, and your body will digest whatever the yeast don't. A classic brewing anecdote is that in the days before pasteurization and reliable city water, orphanages would brew weak beer, >2% alcohol but with lots of sugar and starch left over (i.e. lots of calories) and serve it to children.
The Kefir scoby is more "wild" and wild things tend to be more adaptive, there will be some dominant strains in there doing their Kefir thing when they are in milk, but there will be countless other strains hanging around that will be able to digest all kinds of things, and the mix will change when you put them in wort.