I faced this issue today as well, in a multiplayer server. Killed the old cartographer, created new one in his place, and got the same ocean explorer map. Surprisingly, when I tested it in creative mode, the issue didn't come up again. Test was exactly same - replace cartographer, and trade until he sold ocean explorer maps.
Specifically, in creative mode, whether or not I explored the ocean monument, I could replace the cartographer and get a different map. But, in the multiplayer server, I ended up visiting the previously-cleared monument. And after multiple attempts at getting a new map, I gave up on getting a new monument.
Both used Minecraft version 1.16.5, vanilla; but it may be worth noting that my creative world was ported from previous versions. I don't know if that helps. The multiplayer server, however, is only a few months old, so probably not ported.
Additional information: I previously received a different explorer map when I traded with another cartographer, about 200 blocks away.
I'm okay to run more tests if needed, to reproduce the issue
Darshan Varier
I faced this issue today as well, in a multiplayer server. Killed the old cartographer, created new one in his place, and got the same ocean explorer map. Surprisingly, when I tested it in creative mode, the issue didn't come up again. Test was exactly same - replace cartographer, and trade until he sold ocean explorer maps.
Specifically, in creative mode, whether or not I explored the ocean monument, I could replace the cartographer and get a different map. But, in the multiplayer server, I ended up visiting the previously-cleared monument. And after multiple attempts at getting a new map, I gave up on getting a new monument.
Both used Minecraft version 1.16.5, vanilla; but it may be worth noting that my creative world was ported from previous versions. I don't know if that helps. The multiplayer server, however, is only a few months old, so probably not ported.
Additional information: I previously received a different explorer map when I traded with another cartographer, about 200 blocks away.
I'm okay to run more tests if needed, to reproduce the issue