Tested with an isolated build with create as the only mod installed. IMO I feel like a silk touch enchanted harvester makes more sense for this type of action.

-Minecraft 1.16.5
-forge-36.1.16
-create-mc1.16.5 _v0.3.1c

Have you actually set it to left click mode (through r clicking the front face with a wrench)? I had a similar system with quark cave roots that worked (was in a previous version though)