Any changes being made by players to their maps are vanishing on restart with version 4.2.0.

Either adding new ones, deleting maps, or renaming them, it works fine until the server is restarted and then they're back exactly how they were previously.

The map items themselves do still exist and can be viewed if you happen to know the mapid and give yourself one, but are gone from the map menu (and if you generate them again it uses new map IDs).

Running ImageOnMap 4.2.0 and Paper 1.17.1

Could you try with the 4.2.1 that I will try to release asap (that include a change on the loading of map render, that may solve your issue).

I have tried to redo your issue and didn't managed to recreate it can you try this version ? https://github.com/zDevelopers/ImageOnMap/releases/tag/v4.2.1

Is v4.2.1 only compatible with 1.18? I've just tried it and got a lot of NBT errors with splatter maps. If it's supposed to still work on 1.17 I can submit a second issue for those if you want?