Interesting. I have display prints made with MCC, MCP and Fortezo, all displayed in living rooms, hallways, galleries and lobby of a playhouse. They don't stay in galleries for more than a month or so but others do. None of them changed anything. I also kept some scrap prints in direct sunlight but never seen yellow.
Usually, the paper base turning yellow is a sign of residual silver compounds. But this is not very common with fresh fix, good fixing time, and reasonable wash, especially with RC. It is more risky as the fixer gets exhausted.
Another possibility is the loss of optical brightener used in the sizing layer. In this case the paper loses the bluish tint and becomes more creamy white but not like yellow stain.
Either way, some microscope observation and simple chemical assay would tell what kind of yellow it is...
So back to the original question, was the print fixed in a fresh fixer with good agitation? Was the print washed adequately? (well, 30 seconds may be a bit too short, even with warm wash water and vigorous agitation.)