Update: I fertilized and watered lightly this morning after posting and it seems that the yellow spots have spread to another leaf by this afternoon. Whether or not this is related I do not know. Also, I've noticed that some of the leaf tips are curled downward.

I have never seen this degree of necrosis in Magnesium deficiency, so it leads me to suspect that something else or additional is at work. Any further thoughts? Thank you in advance.