Greek mythology? But seriously please elaborate for my less educated self.

it tests syllogistic reasoning: Jason's mother was Tyro, whose father was Poesidon, whose father was Kronos. it also tests whether it "eagerly" rather than comprehensively considers something: a maternal great-grandfather could be the father of either one's maternal grandmother or maternal grandfather. so the answer could also be king Aeolus of the Etruscans.

ideally a model would be able to answer this accurately and completely.

I think there are more possible answers? Jason's mother differs depending on the author...

For example, Jason's mother was Philonis, daughter of Mestra, daughter of Daedalion, son of Hesporos. So Jason's maternal great-grandfather was Hesporos.

LLMs often don't do well on tasks that require composition into smaller subtasks. In this case there is a chain of relations that depend on the previous result.

Users don’t care about how hard something is for LLMs if they receive incorrect output.

It's categorically more than a weakness.