Nolina recurvata? Closest picture I can find.

10 year old.

Full screaming hot morning and afternoon sun indoor houseplant (atrium
at a College) Yellow spots on old growth, no pots on new growth. Yellow
spots are block shaped like the parallel veination. Spots range in size
from 1/8 inch to 1/4 inch.

No thrips, mites, mealy or scale. Minimal fert. in spring. Dishsoap,
alcohol and lemonjuice to wash leaves occasionally.

Whats its problem? Any ideas?

Thanks for your advice

Susiecatt