Cartographer keeps giving you the same woodland/ocean explorer map even if you already explored that mansion/monument

Whenever I trade with a cartographer, I get an undiscovered map. I go out to discover the map. When I trade with the cartographer again, it gives me the same map even when the mansion/monument was discovered. If you already discovered an explorer map, The cartographer should give you a new undiscovered explorer map. Not only that, but sometimes the icon for the mansion or the monument wouldn't even appear on the explorer map.