I first raised concerns about this issue a little while ago on the sub-reddit (doesn't need to be read if one hasn't). Since then I've done a little testing by going into various worlds in Creative Mode, spawning villagers and trading with Cartographers. This is what I've found:

• In Normal worlds, Cartographers will offer both Tier 5 Pathfinder Map trades.
• In Large Biomes worlds, Cartographers will either offer an Ice Spikes Pathfinder Map as a Tier 5 trade or will not allow you to unlock Tier 5 trades.

I would test this further by trying to /give myself a Mushroom Island Pathfinder Map in a Large Biomes world; I don’t know how to do this, however, so I don’t know whether this issue is to do with the Cartographer or the Map itself. Still, I hope that what information I have delved up is useful!

So, the reason this happens is because cartographers look for biomes within a reasonably large range of where they are. And so, it might just be that those biomes don't exist within... hundreds, I believe, of the villager. Since anything else would be extremely expensive... Closing as works as intended, sorry.