Sadly this's just how the system works, though in my experience it's pretty rare that they release U space one by one in such a short period.

One thing worth noting: I always thought that having F seats next to each other also impacts upgrades for reservations with 2 passengers, but last year we've had multiple instances where the 2 of us were upgraded together into the last 2 F seats that are separate. From your experience, looks like now the only thing that would stop higher-priority elites from an upgrade is U space being released one at a time.

Personally, I do split the reservation whenever possible, but for situations like the BOGO sale, or the BoA companion pass, or connecting from an international flight on another airline, we can only pray that the upgrade processor doesn't do this.