The scenario you’re describing also makes sense, but that’s not what was happening in this case. Most downhill stretches lasted 30 seconds or less in my case, so the battery never reached past 75% full, whether I coasted or gassed and banked.It makes sense if you think about it. If you didn't accelerate and depending on how long of a downhill stretch your battery would become full quickly. Once it becomes full your engine fires up to burn off the regenerated energy instead of storing it in the battery. By accelerating when needed you're preventing the battery from reaching max capacity and allowing yourself to coast down in EV mode as long as possible.
After observing the fuel economy reading for a few months I eventually realized that the most emphatic MPG gains happened after stretches of downhill + regen, so I experimented with artificially making that happen whenever I could and that was the result.