Not sure if this is a bug, or an unspoken change coming to 1.18. But When Caves And Cliffs is switched on, the Birch ColorMap gets completely ignored. I've Attached Two Photos, One Being what Birch Trees look like using my resource pack in 1.17.11 without Caves and Cliffs toggled, and the other being what they look like after Caves and Cliffs is turned on.

To Recreate, Use a Texture Pack that changes leaves colors with Caves And Cliffs Toggle Active.