I had a similar situation a few years back. It was a short flight (AMS-BUD) so it wasn't high on my list of things to fix. When we got to the airport, there was a long line. I asked an agent standing among the people if there was a ST+ line, and she yelled at me that I needed to wait with everyone else. At the time, I was DM. She waived her arm and proclaimed, "All these people are Diamond. You'll have to wait." By the time we got to the front of the line, she was the check-in agent behind the counter. As she took our passports, I noticed what seemed to be a supervisor standing a few feet behind her. I waived over the supervisor, explained and the supervisor started telling the agent what to do. Eventually, the supervisor just moved the agent out of the way, apologized, and UGd us to first, which is just a row of seats with nobody sitting in the middle seat. There's a giant cushion instead.
Right before boarding, the same gate agent, who was now at the gate, called us to the front and told us even though we were sitting in F, we were not getting meals. That really didn't matter to us, but I asked for the supervisor just to make sure that was the case. Of course it wasn't. We didn't see that gate agent again before boarding the flight.
Long story short... There just seem to be some KLM people who go out of their way to make life difficult for DL passengers. It's just a matter of finding the right person and your problem will be solved.
One more thing... My friend I was flying with learned the terms "DYKWIA" and "kettle" that day. She still uses the terms, although she didn't like seeing me as a DYKWIA, even if I did so calmly and was right.