ElasticSearch post_filter and filtered aggregations not behaving the same way
Mystery solved! Thanks for your input, it turns out that the version I was using (6.1.1) has a bug. I don't know what the bug exactly is, but I've installed ElasticSearch 6.5, reindexed my data and with no changes to the queries or mappings, it all works as it should!
Now, I don't know if I should submit a bug report to ES, or just leave it, seeing as it's an older version and they've moved on.