The worldborder plugin appears to be set to 4500×4500 in the nether on the survival server. nether portals beyond 4500 in the over world do not work as expected. when you use a netherportal, i believe you change worlds, then are teleported to the location that is roughly 1/8 of where you were in the overworld. what is not clear is where you are you go initially when you first change worlds. if when you changed worlds (to the nether, using a nether portal) you went to the coords you were at in the overworld before being tp’d to the correct coords, then that would cause the world border plugin to intercept you (which is what is happening). If what is described above is the problem, then the fix to it would be to make the imposed border by the plugin larger, (the same size as the one in the overworld). Doing this may cause other issues though, such as the exact same thing happening but in the opposite direction – if the worldborder set in the server.properties file differs from that imposed by the plugin.
Just a note should anybody read this pose in the future, to say fixed … kind of. It’s a problem with the plugin, not translating the nether co-ord correctly from the overworld coords and tp’s them to the map edge of the nether.