I looked at the tradebanned Pokémon and noticed that shiny 3 Segment Dudunsparce is banned. Why? It could just be that the person is lucky with their eggs. I am making a guess, but just because there's a chance it might be hacked, doesn't mean it is.
#3-Segment Dudunsparce
oak wagon
spice ridge
the odds of shiny are 1/4096 and for it to be 3 seg is 1/100 so while possible it is still very rare
maushold is allowed cause the family of 3 can be found in raids
3 seg dudunsparce can't be found in raids
dunsparce from raids can evolve into 3 seg but it is still 1/100 chances
wispy swift
It's because of the rarity that is why it's trade-banned. As said above all those reasons but as far as breeding, it depends on how you're breeding and the odds you're working with.
The Dunsparce doesn't change it's odds of being 1/100 for 3 seg, if you're using the very best odds so masuda, shiny charm which is 1/512 you're looking at 1/51,000 for it to be 3 seg. And that's if you're not breeding for IVs in particular. You're only ways to get one ontop of it being shiny is literally RNG being kind to you. Breeding in itself the odds aren't guarenteed for shiny either as RNG can be heavy in breeding and some folks go a month or more before seeing a shiny. That and it being rare does unfortunately make it a target for hackers/genners. It's much easier to protect the community as a whole by doing a blanket-ban on it.