No idea why, this happened to me and another user in two different modpack instances entirely. On first mod load.

The only way something like that could happen inconsistantly is through a race condition where some other coremod that modifies BlockLeaves is being initialized before passableleavescore, which would result in the core mod not being initialized at all.

In any sense, there isn't anything in PassableLeaves that would cause this. If the coremod doesn't get initialized, then the reason should be stated in the log files.