Like their rhododendron cousins, camellias have a very shallow, dense and matt-like root system. They dislike being planted too deeply and they dislike being smothered in mulch. 4 inches is excessive.

It's not a matter of the camellias preferring the mulch. The roots grow into it because they can more easily access necessary oxygen, water and nutrients. Were the mulch not present, the roots would have done the same colonization of the surrounding soil.

The recommendation for mulching around these plants is to have the greatest depth of mulch at the dripline and then feather the mulch gradually back to the base/trunk. And once again, 4 inches is on the excessive side. 2-3 inches is sufficient.

@gardengal48 (PNW Z8/9) Now that I am where I am, how should I remediate the situation? Because now I effectively have no/minimal mulch, and the roots are stuck in dry soil with little nutritional content. I could maybe use a rock mulch and add some organic soil above the old mulch?

There is really nothing that must be done. The mulch is still present and doing what it is supposed to do, even though permeated by roots. And a good portion of the camellia roots are also present in the raised bed soil so able to access whatever water and nutrients are present there. Camellias are not heavy feeders so unlikely the shrub will require much in the way of supplemental fertilization. I'd just keep an eye on any watering needs.

I've had lots of plants root into mulch. They don't seem to mind. In many cases the mulch is really good at absorbing and holding water, so it helps the plants stay hydrated.