I flew this past December and early January on the return on KL. Award ticket was DL out (KL metal) and KL ticket on the return. Was able to go to KLM.com and do seats for the return including EC selection. I was amazed that my ST EP status was recognized.

That said, I believe the issue is less DL's IT and more the fact that KL is charging for EC seating. DL showed the EC section as light-blue (elite section) so you can try and grab em but they're not yours.

Great information and FWIW, the EC seats really do have more room and double recline. I would suggest not taking the last row in EC in that while I know it has the added legroom, I wonder if it has the double recline.