edifying a tree executes successfully but the sapling remains unchanged.

I spent some time poking at this one. It fails because the edified trees are never registered as ConfiguredFeature<?,?>s.

However, simply trying to register them in the same manner as other registered objects failed.
On Forge, the registration method never got called. On fabric, there is no BuiltInRegistries.CONFIGURED_FEATURE

hexcasting-fabric-1.20.1-0.11.1-7-pre-609 and hexcasting-forge-1.20.1-0.11.1-7-pre-609
As original post states, the spell executes when given a sapling's coordinates, yet doesn't do anything to the sapling.
To replicate, cast the hex on any given sapling

Potential fix submitted as PR #594 .
I'm not happy with that solution, but it does serve to demonstrate exactly what's wrong.