Ok, explains our HEL-AGP-HEL flights. For the departure in December I was able to select exit seats fine but there was a fee for the return flights late January. I thought it was yet another AY IT glitch but didn't have the time to call AY yet..
I think this is one of the small perks that actually benefit the airlines, happy to pay a bit for more for intra-Europe AY flight with the "free" (like airline status was free..) exit seats vs. perhaps cheaper flight on some other carrier.