I guess the idea that the bacterial strains mutate in ways that will produce flavors we may dislike comes from the idea that when wine makers use a variety of yeasts in the same batch they should not attempt to harvest them for reuse because although they may have decided to use say 1/3 qty of each of 3 yeasts for the total amount of yeast they pitch, different yeasts may dominate and be more powerful at creating the environment they prefer so that after a single use the 1/3 -1/3 - 1/3 may be 1/2 -1/4 - 1/4 or even 3/4- 1/4 -0 and so the subsequent batches may not taste anything like the first batch and the same problem MAY occur with bacteria that constitute the yogurt. I may use three different cultures applied by the yogurt company but I have no idea whether these three cultures grow together or whether they are combined when a new batch is being made, and kept absolutely separate at other times.. If the latter, flavors may indeed change and I have no control over which , if any bacterium dominates..