I use OTG with Biome Bundle for my world type and when the season changes to Autumn a ton of the leaves randomly decay for no reason. Even on normal trees that are player grown I see this happening. Why do the leaves decay, and can it be stopped?

Well, I don't use OTG anymore and I no longer see this issue. It probably has to do with all blocks suddenly updating and the game realizing that some aren't close enough to any logs.