There are several possibilities.

I think the most likely is that the original color was what is called a "sport". A sport is a vegetative (not seed) mutation.

Every once in a while, a gardner or plant breeder will notice that one branch of a plant looks different. Maybe the flower color is paler, or the leaves variegated. What the grower might try to do is take a cutting from that branch, and propagate it.

The funny thing about sports is that not ALL of the cells are mutated. Originally one and then a few cells in the shoot's bud had the mutation, but not all of them.

One famous sport is the Lavatera called "Barnsley". Rosemary Verey either found it or someone gave it to her--I don't remember the story. Anyway, it has pale off-white pink blossoms, instead of the usual deep rose blossoms.

Now then, what tends to happen over time, is that the mutant cells that produce the pale pink blossoms are not reliable; sometimes they die out! Eventually Barnsley tends to revert back to the plain old deep rose.

That's probably what partially happened to your Fuchsia. The color variation--the "sport"--died out in one section of the plant.

If you want to save the original color, take starts from the branches that still produce flowers of that color.