It doesn't look lke sunburn. Were it sunburn, the color would fade from colored to neckrotic in a fade...not half the leaf green, and the other half brown.

Leads me to believe there is a root problem. New growth up top looks ok. Is it ok...? Perhaps the roots got steamed or toasted. Was it sitting on a hot sidewalk or patio?

What are you using for potting soil, and where is the perlite to make the mix 'lighter'? Heavy soils compact around the roots, suffocating the plant, and can easily burn roots, especially if ph is off.
Without perlite, composts can and will get 'muddy', and tends to retain water too much or conversely, can 'cake-up' and not absorb water at all.