As far as I know, you can keep mounding soil around potatoes until the frost takes them. I’ve heard of guys raising potatoes in beds made of tires, and had to stop stacking tires long before the spuds stopped growing. In theory, the deeper you mulch them, the more potatoes they will yield.
Keep ‘em watered...
After I watered them yesterday, we got a huge rain last night! No rain was predicted and there was not a cloud in the sky yesterday. I mowed all day (over very dry ground, which is why I set a sprinkler out in the veggie patch), and then came the deluge. I’m sure it doesn’t matter. Will add my last 2 bags of garden soil this afternoon. If it doesn’t work, it doesn’t matter. The soil needs to go into that bed anyway.