I still have this problem.
when i join the server the difficulty is hard. then when i travel to the nether the difficulty is still hard but when i travel back to the overworld the difficulty automaticlly changed to easy. When i do /difficulty hard it changes it back to hard but when i travel to the nether and then back to the overworld the same thing happens.
i tried to use the plugin multiverse but i didnt find any video properly explaining how that could help