The vegetation_patch feature has a ground_state field where blocks that are placed under it can be specified. When using a weighted_state_provider (issue may persist with other multi-block specifiers, I have not tested others) to specify multiple blocks the probability of which blocks are placed can be inconsistent. If one of the blocks is already present under/above (depends on if its on the floor or ceiling) the feature before it is generated, when the ground_state blocks are placed, the block that was already present is never placed, leaving it to chose from the remaining blocks.

I'm using vegetation_patch features to generate "crystals" and for the attached images, the ground_state is a 50/50 for deepslate and deepslate_redstone_ore, when the feature generates on stone, it places the mix of blocks as intended, but when the feature generates in the deepslate layer, the feature only places the deepslate_redstone_ore. Issue is not limited to these blocks. (Attached datapack uses different blocks, same effect)