Structures that use stone blocks may generate with ores and other stone types

I recently wanted to add a custom structure that utilized stone blocks but these structures would occasionally generate with ores and other stone types (including gravel, dirt, and other blocks that generate under the surface) in place of the intended stone blocks.

I initially thought this was an issue on my end. I changed the 'plains meeting point 1' structure to use stone and added that to an otherwise empty data pack in a single player vanilla world. I searched around, and found what is mostly a granite well instead of the intended stone well.

I know this likely occurs because the population of foliage and ores occur after the terrain generation, so it makes sense that stone is replaced but I strongly feel that this should not occur with blocks that are part of the structures themselves. 

I couldn't find any information about this, so I'm not sure if it's intentional for structures to have their stone blocks replaced, or if there's an option that can be enabled in a data pack to disable this behaviour.

Update: The 'step' property of the structure determines when the structure should generate. Setting this to 'underground_decoration' eliminates this issue, but it is of my opinion that this isn't perfectly clear cut. I was under the impression that surface structures use the equivalent step. This makes more sense to me now. Not a bug in my opinion, just an ambiguous option. 🙂