This week I haven’t noticed that it is cheaper to buy separate tickets on multi-city trips. Has that changed?

You are referring to the change in airline “combinability” fare rules that occurred in mid-March and prevented many of the lowest fares from being combined for trips with 2 or more stops or going and coming from different cities on American, Delta, and United. Booking such trips with a separate ticket for each leg saved hundreds of dollars. Fortunately, due to massive outcry from the travel industry, this has changed. The airlines have returned the rules to the way they were before mid-March and there is no little or no difference in cost whether you book your trip as one ticket or several tickets. There are a few exceptions, most notably when one leg of the trip has an exceptionally low fare due to competition with a low-cost carrier.