The bug

Mossy cobblestone boulders in old-growth-spruce-taige biome appear to generate in a grid-like pattern.

To reproduce

Option #1
Seed used:

8100499608039829553

Coordinates:

/tp 999763 ~ -1998077

Option #2

1. Create a single-biome world and select "Old Growth Spruce Taiga" as the biome.

2. /give @p minecraft:repeating_command_block

3. Place the command block and set it to "Always Active".
4. Put the following command into the command block:

   /execute at @p run fill ~-15 ~-18 ~-15 ~15 ~15 ~15 air replace minecraft:spruce_leaves

5. Fly around and look at the boulders. (The command will remove leaves for you so you can see more easily.)

Expected behavior

The boulders should be placed randomly.

Observed behavior

The boulders are in a grid.