Minecraft can fail to generate leaves textures, causing the texture to turn black.

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I can confirm. However, this most likely is intended due to the fact that the attached resource pack explicitly requests:

mipmap_strategy: dark_cutout

this intentionally blends transparent/cutout pixels so that edges and distant mip levels look darker, this was documented in snapshot 25w43a's changelog:

dark_cutout is similar to mean, but blends colors in a way that makes the pixels bordering cutout pixels darker. This was and stays the strategy for leaves and Mangrove Roots