I have several ocean explorer maps which were purchased from a cartographer. These maps all used to point to the same ocean monument, which was located in the second map created in the previous game ("map 2"). After fulling exploring the ocean monument map, it was effectively the same map as "map 2", and for that reason, I placed the ocean explorer map in an item frame to complete the previous game map room achievement.

After old world sync, the ocean explorer maps initially remained as duplicates of "map 2". After removing the ocean explorer map from the item frame, it turned into a new ocean explorer map, which now points to a monument in the 10th map. "Map 2" is still map 2, but the ocean explorer map is now effectively "map 10".

This is in a world in which all 9 maps were previously fully explored.

 This is related to MCPE-34698.