The failure you describe at the end is exactly mine and I want to name why it is so hard to spot.
A stale memory does not produce obviously wrong output. It produces slightly-off output that is completely defensible in isolation. Every individual suggestion looks reasonable, so you correct it and move on, and there is no single moment where you think "something is misconfigured". The cost accumulates as a vague sense that the tool has got worse.
What finally caught it for me was reading the stored list cold, months after writing it, and finding three entries I disagreed with. Not subtly - I actively did not believe them any more.
So the practical thing is your monthly prune, and I would add: read the entries without the context of why you added them. Anything you cannot immediately justify on its own terms should go, because "I probably had a reason" is how stale facts survive.