Spectre leaves have the decayable tag set to true when placed down by a player. It doesn't seem like the leaves actually despawn unless you mine a leave block or any log block that might be attached to it.
The log and leave blocks don't actually have to be spectre wood/leaves, in fact they can be every log/leave (tested with Vanilla and Extra Utilites 2 logs and leaves).
Would be nice if that can be fixed to allow for easier building with the leaves.

Mod Version: RandomThings-MC1.12.2-4.2.7.3