Why are there different pronunciations of 复杂？
The reason is simply because it's 顺嘴(for better fluency/flow better).
It could be that we leverage it from old generations.  And then we have been corrected in school.
In practice, we know the correct pronunciation for it, but sometimes most of people still say fu3za2 occasionally because they already get used to it.
I don't think this only applies to the southern areas.  I also find it's been said by people from the north.