When decoding an event like Foo(uint256 a), I end up with an object like:

{ /*...*/, values: { a: { _hex: '0x5' } } }

I would have expected a BigNumber here, or barring that a hex encoded string. _ prefixed property names are usually internal, so I am hesitant to write code that relies or reading that. Also, non-number values are stored without the inner-most object, so just { b: '0xabcd1234' }, which makes number values odd.

After a significant amount of digging I found the issue is in the way it "freezes" the values key. It does JSON.stringify then JSON.parse on it, which results in stripping the prototype from all of the methods and leaving only the _hex internal variable of the original BigNumber object.