Yep. As mentioned
Not all hacks works all the time. I thought that hitting attributes.country would do the same as your equality on JP. May be you could try with some other tests that would hit the same field but with more specific values compared to null. I would try with
"attributes.country" : { "$gte" : "AA" , "$lte" : "ZZ" }
Assuming your data follows the ISO-3166 2 letters country codes standard, all countries should match. Hopefully, this range query will select the same index as the equality query of JP.