Upstream servers are not reintroduced to the pool after becoming availabe

[Fry-kun]

My experiment: I've set up 3 upstream servers in default fair mode.
Made a few queries to confirm that happy path works - servers are queried in round robin fashion.
Took down 1st server. The next few queries were sent round-robin to 2nd and 3rd, as expected.
Returned 1st server to operation. Queries continue to only go to 2nd and 3rd servers, not following nginx fail_timeout settings (tried both default and several seconds).
Took down 2nd server. All queries started going to 3rd server only.

It seems that when the traffic is low enough to not cover all "working" servers, the "dead" servers will be completely ignored - this is very different from nginx behavior.