From googling this a bit, it seems like the combat log only auto-scrolls as entries are removed from it, or something to that effect. From trying to review an Overwatch case where someone was reported for suspicious movements/casts, I can't see it auto-scrolling at all.

What's annoying about this is I'm trying to watch the reported player's cast timings during the reported sections in an attempt to find repetitive item/ability usage timings, but the combat log won't auto-scroll as the replay progresses. How am I supposed to watch the combat log and the replay at the same time if the combat log doesn't scroll to show the most recent entries? This makes no sense to me, and I would love to know if there's some way to enable this or something that I'm missing.