I think it’s because you search in:first. The post you get as a result is not 19h old, it seems to be from 2017.
But I understand your problem. If searching in:first there is no difference between sort by latest post and sort by latest topic.
I don’t think this is a bug? If you use the in:first filter by latest it will give you the results of the first post in date order. This seems to be working as expected?