Wait, there can be muliple people on one resource? Yeah, I definitely didn't figure that one out. Though, I did eventually discover that the houses do in fact generate people, there's just no UI showing the progress.
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Who knows XD. I did manage to complete it with only one worker per mine.
Just had a little look to see what happens. I don't think you're suppose to place two workers on one mine; the two sprites overlap and the output seems to be the same. Perhaps a coding bug, but then it's weird that you can't place to workers on one tree.