Versions:

• Minecraft 1.20.4
• Fabric Loader 0.16.7
• Chisels and Bits (for Fabric) 1.4.157

If I place Leaf Bits (Edit: it's not just leaves, but every block with transparency), they render just fine as long as I don't add an Bits of Blocks that don't have any transparency. But if I do, the Leaves start to render opaque. The preview of a created pattern render fine though.

Example:

On the right, how it renders, when placed.
In the middle, the preview the pattern shows, when holding it in the main hand.
On the left, how I expected it to look (Custom Model made with Blockbench)