Are the leaves changing color before dropping? If they are changing from green, can you describe how they change?

Are you waiting for the top 1-2" of soil to dry or almost dry out before watering again? I am trying to figure out if you are having root rot due to too much water.

Leaves do not change color before dropping. They look green and healthy. Maybe I can notice veins on them a bit more than when I just bought it, but I'm not 100% sure. Would the leaves change to yellow if it were root rot?

Normal leaf replacement occurs near the end of Spring and makes the leaves turn yellow before falling. But I doubt that a newly purchased plant would be doing that in September...

Root rot prevents leaves from getting moisture so the leaves would react in the same way as they do when they lack water, meaning they would slowly turn brown. Too much sun can also turn the leaves brown. But your camellia leaves are falling while green. Hmmm... I am going to ramble things in no particular order...

So now you have a large number of green leaves at the bottom of the bush that are turning brown?

You see no pests (like scale, etc) on the leaves or in the branches?

How does the soil feel if you insert a finger into the soil to a depth of 4"? Wet, moist, almost dry or dry? How much do you water (in gallons if you know)? And how often do you water?

Can you check if the shrub is planted too deep (in relation to the surrounding soil)? They tend to have problems getting oxygen if planted too deep.

Is the pot getting a lot of heat from the sun? I assume the pot is getting afternoon shade and is not in a windy location. Can you confirm?