All marbles, whether koi, nemo, galaxy, etc will most probably shift patterns. Some may even change colors. People are saying, marbles with large color pattern, such as nemo, are more stable (I can't confirm since I stay away from them).
Only very few have bred out the marble to make betta's colors/pattern more stable. But most haven't since it's quite difficult to do. Plus they may lose the pattern if they breed out the genes.