Universal is much better about single rider, of course, but far from perfect. Last weekend at IoA, Hulk and Veloci were dispatching with empty seats frequently enough that it caught my eye, while singles were standing right there in queue. I was in such a row twice myself, and I think I only took 7 coaster rides in total that day.
One consistent element of these partial-row dispatches at IoA, anecdotally at least: interruptions.
In one case when I was in a partially empty row (Hulk), a guy got walked for some reason (possibly being over size). After focusing on that instead of having an op free to look for singles to fill in rows, they immediately got the restraints checked and sent the train. In the other case (Veloci), some kid dropped his hat between train cars on the boarding side, and it landed on top of the rail between wheel assemblies. Bad idea to dispatch in that condition. Ops followed protocol and called maintenance, and the responding guy with mild annoyance reached down casually and grabbed the hat off the track. Then they quickly boarded us and sent the train, with seemingly little or no attention paid to the single rider line. Single rider seems to be the first thing to get deprioritized in such cases, which aren't as rare as one would hope.
Meanwhile at BGW, if you're operating a ride that apparently has frequent restraint issues in the station, and op training and motivation maybe aren't quite as crisp as at Uni...