Why are the lower leaves of my Variegated Japanese Aralia / Fatsia Japonica dying?

Several months ago, we planted a Variegated Japanese Aralia / Fatsia Japonica in the ground. It gets a little morning sun, but is mostly shaded all afternoon. It’s grown quite a bit and the upper leaves look quite healthy.

However, it seems like several of the lower leaves are dying off. Specifically, they turn dark brown, become dry and brittle as if they were baked in an oven. They eventually break off where the leaves join the main "trunk." I don’t think it’s getting too much sun since the lower leaves are shaded a bit by the upper leaves and the upper leaves show no signs of being burned.

Shown in the photo circled in red are what’s left of three leaves, parts of which have turned brown, dry, and brittle, and just crumbled off. For other leaves no longer on the plant, if they manage not to crumble off, eventually, the entire leaf and stem will drop off.

Is it normal for the lower leaves of this plant to die off like this? If not, any ideas what’s causing it and how to fix it?