From memory, I think this may be due to the fact that the agent expects an array.
Can you try like this?
defaultValue: async context => [await context.getRecord(['familyId'])).familyId],
This was coded like this to make it possible to handle composite ids, but is clearly counter intuitive.
If that works, I’ll add a ticket to either improve the doc or improve the API (or at least, throw a proper error…)