Furthermore, they aren't flammable.

This seems to apply to all leaves from this mod.

Can confirm they neither decay nor catch on fire. I did test a lot of Tropicraft blocks and it appears not being flammable is by design.

Flammable issue has been fixed but not released yet, the decay part is a more complicated problem...

Not only do they not decay, but the leaves do not drop any saplings when broken

As it is leaves not decaying is "intended" (as otherwise they'd decay upon trees being generated, we'll be looking into rewriting logic for that for 1.13/1.14). However, saplings not dropping is definitely a bug. Could you open a new issue for it? @theBudster