It is a mutation or form of reversion. Not at all uncommon with camellias.

Such natural mutations, where considered attractive, can result in new cultivars which are then propagated vegetatively.

Variegated camellia cultivars tend to have reversion and produce unusual blooms at times. Seems as if some of those varieties do not have a stable DNA. My normally white with stripes Eleanor McCown suddenly had non-variegated, full pink blooms from one branch. Then the branch developed an unusual reversal of white-pink colors. Eleanor McCown is normally white with pink stripes but one bloom in the branch was mostly pink with white stripes. You can cut off branches that do that if you prefer. Viruses in camellias can sometimes form variegated blooms.

Looks just like my Governor Mouton camellia. What the others say is correct - camellias are known for their "sports" or mutations. At another house I had one camellia bush (probably 80 years old or so) with at least 3 unique variations on the same plant!