I created a blueprint for a full furnace stack along with the copper ore miners and a few coal ore miners.
But when I place either all miners get copper ore including the ones that were with coal or the other way around.

Do you have the startup setting Blueprint Resources enabled?
When the startup setting is enabled, and you then create a blueprint, that blueprint should save the different resources.
Otherwise, blueprints will place the resource chosen in the Resource selector.

To further clarify, if the Blueprint Resources setting is enabled and you then make a blueprint, the resource is saved in the blueprint. So it is possible to enable the setting, make a blueprint, then disable the setting, and that blueprint will continue to place the saved resources. This is true in the opposite. If you make a blueprint when the setting is disabled, that blueprint will always follow the selected resource, even if you then enable the startup setting. The saving (or not) of the resources, is bound to the blueprint at the time it is made.

Oh, you're right, I haven't enabled it.
Oops.
Sorry for the waste of time.