This might be intentional (though I really hope it isn't), but I figured I'd post it just in case.
Rainforest trees placed on the craftable Dense Turf cannot be "infected" by nearby Cocooned Trees, because spider_monkey_herd.lua line 52 only counts the natural-spawning deep rainforest turf as valid. This makes it impossible to have Cocooned Trees outside of the deep rainforest, for those of us who wanna use them to make our base pretty.
Steps to Reproduce
spider_monkey_herd.lua > line 52
spider_monkey_herd.lua > line 52
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.