I think the problem is that, in the first sentence, "thrived" is incorrect because of what it implies. If you say "thrived" then the action has finished and it would be assumed that the sheep no longer "thrive" there. This would not make sense in the context of the current sentence. Alternatively, you could replace "thrived" with "thriving" and it would be perfectly fine, since the word "thriving" shows an on-going action.
For the verb "positioned" in the second sentence, it makes sense, because the verb "to be positioned" is a verb that shows a particular state; one that, when finished, remains that way. The action is no longer occurring, but the result of the action (the state of being positioned) is what is left.
I hope this makes sense. If my explanation is confusing at all, please let me know and I'll try to re-word it, but for now this is the best way I can think to explain it.