Hi BonsaiNut... its not unusual for a variegated plant to lose its variegation... often the variegation exists on a plant as a genetically distinct 'island' which is weaker and less competitive than the wildtype islands of the tree. Even if a variegated plant is started from a cutting, there are likely still islands of wildtype cells on that cutting.
If/when a wildtype shoot emerges on a variegated plant, it will eventually outcompete the variegation and take over the whole tree. While the island of variegation may still exist somewhere on the tree, it is not competitive enough to bounce back.
So, if you want to encourage this bit of variegation to thrive, I would consider defoliating the wildtype leaves near it and allow it to gain some strength, but as long as there are wildtype shoots on the same plant, the variegated sector will likely never take over...
Cheers