The rubber leaves block is registered without a state mapper. Because of this, visually irrelevant states (check_decay, decayable) are considered when resolving ResourceLocations. This leads to a model loading error, log noise, and - what interests me most - Better Foliage being unable to determine the texture for the block model.

Please check how vanilla leaves are registered, that is the correct way.

Using TechReborn 1.9-1.1.0.258 and RebornCore 1.9-2.2.0.77.
I didn't specifically check, but the 1.8.9 version most likely has the same problem too.