When I grew mine it was a very heavy bark mix, good drainage is the key

They like to be in shade, unlike other Amorphs like A. paeonifolius. Also, when they go dormant, you cannot let the soil medium get dried out as you can with other Amorphs. You will lose it if you do.

Several places say it is native to the ''rain forest''. Just curious. Do rain forest have ''good drainage''?

To answer the question about "good drainage", you need two things. One is the conditions in the forest. Virgin rainforest floors will have layers of fresh and partially decomposed organic matter which is very quick draining. The way Amorphs grow, the seeds would fall into and germinate within this mix of leaf litter, twigs, and partially decomposed matter. So the great majority of the root system would grow horizontally in that zone, while the tuber would form deeper in the mix, eventually reaching the subsoil, which would not be as well draining as the near-surface zone. Besides that, the plants may also be growing on a slanted substrate, such as a hillside, which also facilitates runoff of excess water. Second is the differing conditions in the pot, which can be very moist to wet when compared with natural conditions, due to the artificial nature of the medium and the fact that the only outlet for excess moisture is/are the hole(s) in the pot.

"Rain forest" isn't just one specific lot of conditions, it covers a variety of situations, depending on things like soil type, altitude, topography, sun exposure, and of course drainage. These can vary a lot within a very short distance. Some parts have bare soil, some parts there's an accumulation of leaves and organic matter. Plants find their own little niche. Seeds that fall randomly end up in either ideal spots, or not. If they're lucky they grow on, if not they sprout then wither and die. Probably most seeds in nature don't get the chance to develop into mature plants, they end up in the wrong place, even though not far from where the parent plant grows well. Seeds of A. titanum that fall into creeks/rivers or get washed into swamps just die. For them ideal conditions occur mostly on slopes and higher ground.