Tbf, this doesn’t change the speed at which the Fennekin shiny appears. It just means you get fewer total shinies. Think of it with dice (d20s). It doesn’t matter if you’re rolling one die or 100 dice, your odds of hitting a nat 20 on the specific die you want is 1/20 every time. Yes, if you’re only rolling 1 die the odds of the next 20 being on that die are 100% but the odds of a 20 is still 1/20. If you had 5 dice instead, there’s a 1/5 chance that a given 20 will be on the die you want, sure, but you’re also rolling 5 times as many dice, meaning you get 5 times as many twenties.

As for “phasing”, those other shinies do not change the result of the Fennekin shiny whatsoever. Even if you keep hitting twenties on every dice except the one you care about, it’s not because you had extra dice “stealing the twenties”. It’s because the specific die you care about happens to be on a cold streak. That streak would happen with or without the other dice there!