I don't know if this is a bug or just new operation. In 1.13.2 (and maybe until recently on 1.14.4?), if you enter a nether portal on a world that doesn't have a corresponding nether world, you just sit in the portal and never teleport. With the latest version of 1.14.4 though, you teleport to world_nether. This can be stopped by canceling the PlayerPortalEvent, but it's new, so I'm not sure if it's intended or not.

Example:

Server has these worlds: 

• world, world_nether, world_the_end
• example_world

Player is in example_world, builds a nether portal and enters it. They get teleported to world_nether. If they then standstill in the portal they get teleported to somewhere in world.

I would expect that nether portals should not really teleport across worlds.

This situation doesn't require any particular plugin. It's just behavior I've seen recently.