I suspect its leaves are getting their DECAYABLE property set to false via metas, but haven't checked the code yet to confirm.

Also, having just chopped one down, I suspect we'll need to add some log blocks in a few places, or else most of the leaves will decay straight away because there are leaves way further than 5 blocks from a supporting log block.

The opposite is actually true here... leaves are decaying immediately for some RTG trees, including but not limited to the Swamp Willow. As a quick fix, I've added something in a2b36e0 which sets the CHECK_DECAY property of all RTG tree leaves to FALSE. Ideally, we should be doing this when we're creating the Deco object in RB classes, but this will do for now.

[Edit: I assume the reason they weren't decaying for me is because I was in a swamp next to a Roofed Forest so the low TPS was preventing them from decaying.]