Chasing robber away not possible in turn I activated the knight
Since the robber was on a hex I wanted to be productive, I activated a knight for chasing the robber away. However, this action was not possible during my current turn, I had to wait a turn before I could chase the robber away (and so again could not yield ore from that field while it should).
What you're seeing is correct behavior. Those are the rules of Catan. You can't activate a knight and then immediately move the night or chase away the robber on the same turn you activated it