in 2016 we flew UA SFO-FRA connecting to TK FRA-IST and in fact we did not get our TK boarding pass until the gate in FRA. In FRA we had to do a lot of walking, and we went through a security check point, but it was fine, we never entered the EU and as soon as we arrived at the gate they were waiting for us with boarding passes.

As in - they knew we didn't have boarding passes, already had them printed out and waiting for us. So I think this may be common. IIRC they even asked something like "Are you Mr. Returnoftheyeti? I have your passes for you.

in 2016 very few Americans visit Istanbul so I guess we were pretty obvious.