I experienced the exact same thing. Had articles perform exceptionally well. Frequently 50-100k views with a similar RPM to yours. But around November, the RPMs tumbled.

Took a quick hiatus and came back earlier this year to experiment and see if things had changed. But nope. The more views my articles get, the lower the RPM goes. Just feels like it's not worth it anymore.

I miss the good ol' days of CV scores/fixed RPMs.

NB needs content creators. But they also need to keep the RPM high enough to make it worth it. Otherwise, people are just competing for literal pennies...