It seems that there can't be more than 8k of a ressource mined on a map. Once there are 8k of the same ressource (Brimstone) are in stockpiles they will still mine what they are currently mine but they won't get ressources out of it. After that they just stop working on that task.
This can happen on multiple maps (Apex Arecibo Nova Pineburg Big Island). It is highly unlikley that it will happen during normal gamplay though.
I attached a save file from Pineburg where 8000 brimstone was mined and stored.