I'm trying to create a schematic of these 'posters' on a Minecraft server. They're created using a collection of maps in item frames. When I created the schematic and pasted it in a singleplayer world, all of the maps became invisible and picking any of these with CTRL + Middle Click would give me a map with the description of 'Unknown Map'. Each map from a different item frame takes a separate inventory slot when doing this so they must be recognised as different maps. Any help is much appreciated

Last time I checked I'm pretty sure the maps only store like their ID, scale and center position or something like that in the item itself. The actual "contents and color data" are synced from the server based on the above mentioned details, including the map ID. The contents are stored in per-map files in the data directory.

So this means you can't really copy paste maps across worlds using schematics alone, because that map data file in the world won't exist in the new world, or at least it won't have the original contents. You would also have to manually copy those map files over. And then you may run into conflicts with the map IDs if you try to copy them from multiple worlds where the same IDs are used, or the target world already had maps of its own.