I've been creating custom villages with datapacks, and one odd occurrence I've noticed is that I sometimes see Deepslate Coal Ore and Deepslate Copper Ore randomly replacing blocks in the village roads. It only occurs when the village naturally generates (not when using /place structure), and the datapack never even makes a mention of these blocks. I have a guess at what might be causing this bug: during generation, the game does three things.

1) Identify a place with naturally-generating stone where the game would like to put ore

2) Place custom structures defined in datapacks

3) Actually place the ore it had said it was going to place earlier. Any location this ore might occupy was supposed to be either stone or deepslate, so if the block there isn't stone, it must be deepslate, and therefore the ore should be deepslate ore.

Problem is, step 3 seems to ignore that step 2 happened. Locations that were previously stone might now actually be blocks belonging to a custom structure, meaning they'll be assumed as deepslate (because... not stone).

Again, this is just my speculation as to the cause - all I know for sure is that these deepslate ores weirdly generate at the surface within custom structures.

The attached screenshot is an example of this happening. I'm in a custom village that I created, and there's a block of deepslate coal ore in the middle of the road.

Edit: Upon further testing, I have found Deepslate Iron Ore in my custom structures as well, which would seem to line up further with my theory from earlier.