Hier the same as well. I had changed my outbound seat 1 1/2weeks ago, when more seats opened up on a short hall flight. When I took the flight at no point could I change the seat after that, on any device.
For the inbound I wanted to change as well, also not possible since changing the outbound. What I had done approx 48 hours before departure was make a dummy booking and see what is available, then contact KLM through whatsapp. They replied within 10 minutes and moved me further forward.
They did not know what the issue was.

Also cannot change the seat on an inbound flight from Canada in September, don't know if that has the same issue.