Indicas are very prone to sporting. I've observed it myself in established plantings and in nurseries alike. In my experience, if you remove the sport, it is less likely to come back again. Some sports have actually been the source of new cultivars.

Sporting is a relatively common occurance in horticulture. Fruit culture, agriculture, and ornamental horticulture have all benefited from the phenom.

'mrs. g.g. gerbing' is a pure, white sport of 'george l. taber', so you may have these two on one shrub. they are two of my favorite southern indicas and are both fragrant.
one of my 'george l. taber's has a branch with solid purple flowers each year, very similar to the one in the lower photo. i was thinking the he is a sport of the purple formosa azalea but i can't find any reference to that.

I've also seen solid purple flowers growing on 'George L. Taber.' The American Rhododendron Society says that George L. Taber is a sport of 'Omurasaki' which seems to be a purple like 'Formosa.'

So, Purple 'Omurasaki' sported to pinkish 'George L. Taber' which sported to white 'Mrs G G Gerbing.' Pretty cool.

It would be very interesting to know the genetic basis of the color changes. The fact that they are so unstable suggests that they might be due to a transposable element, or something similar, that can insert and delete.

I had sports this year on my light pink azalea. One flower was half and half!

Somehow "mark" the branch that produced your bloom to see if produces similar blooms in future years.