I was able to close out of the game and reproduce this bug.
My hero does 1 damage and applies 3 overburn, hitting a winter worm with 4 health and 3 overburn. On hit, the visual overburn counter ticks up to 6, but the explosion only deals 3 damage to the other unit in the row. Reproduce by using a berry bell on either row.
Perhaps 1 damage procs and triggers the overburn explosion immediately, and then the 3 additional overburn is added after the explosion damage is calculated?