Standard run, all settings untouched, and about 90% of the resources dropped have been copper.
For comparison, I currently have 90k copper ore available to be mined. I've seen about 150 coal total, 300 stone, and maybe 10k iron?

Biters that die near the existing resources will generate that same resource, so if you have a lot of small copper patches, they will grow into more copper instead of generating new resource kinds. In order to get more resources, kill biters slightly away (>10 tiles) to roll for the resource you want and then funnel biters to that spot to enrich the new patch.

If you believe you have a different issue, please send me your savefile so I could take a look. I'm juh9870 in Discord, or you can open an issue on github https://github.com/juh9870/resourceful-biters/issues with the savefile

That makes sense! I'd seen overlapping resource patches of different types, so assumed that wasn't the case.

overlapping patches can happen when biters die nearby two different patches or when initial biters die far enough apart to create multiple patches that then expand to touch