Searching by term only shows oldest post first:
Search by user and term shows oldest post first:
Searching by user only shows newest post first:
Personally I find that seeing the most recent post first is more useful more often, though it really depends. In the short term, wouldn’t it be best to always sort these in the same order? In the long term, it might be nice to get a checkbox or two option dropdown to let you easily flip between oldest and newest sort orders.