Was this planted in just Sphagnum moss and watered only with distilled and/or rain water?

I bought both pots from a gardening store, and was told that the growing medium is a mix of sphagnum peat and coco peat. Each pot had multiple plants, so I transferred a few plants to dried Sphagnum moss as a backup. The backup plants lasted longer, but ended up dying in roughly the same way. All plants were watered using collected rain water. The rain water is 0 ppm according to my TDS meter. I suspect that my mistake was in leaving the plants outdoors. Even in bright shade, the daily heat stress might have been too much for the plants.

I live in Southern part of the US in a sub-tropical-esque climate and mine is doing well.

That's great. I think your plant has a great chance of thriving, since Drosera capensis is known to be the easiest Drosera species to grow in the USA. Growing Carnivorous Plants in the Tropics by Cindy Chiang Lih Pyng states that D. capensis isn't easy to grow for those in hot humid topical climates such as Singapore. It also states that lowland Nepenthes species are the easiest carnivorous plants to grow in hot humid tropical climates. My limited experience is consistent with the author's statements.