It's neither overwatering nor underwatering: it's both. It's called watering stress and it usually happens when the top portion of soil is bone dry and the bottom is soaking wet. The plant becomes seriously stressed and starts dying off.
The most usual causes are use of an undervase and poor draining. If you really need to use an undervase, fill it for at least 1/4" with gravel to keep the bottom of the vase as dry as possible. Drainage can be a problem with many vases and potting compounds: after many years and too many dead camelias I have found the best way to solve this issue it to put a layer of lapilli or crushed rubble at the bottom of the pot to help drainage, and to mix a little gravel, crushed pumice, vermiculite etc in the growing medium. Everything that helps drainage.