Hey Eg, Im not from mojang but i encountered the same issue on my server. You need to copy over the 1.16 files(Behavior_packs,etc) to your new server so your nether generates correctly.
I encountered the same Issue on my server. You need to copy over all the 1.16 file to the current one. Behaviours and all. After doing that, it fixed my nether.
Hey Eg, Im not from mojang but i encountered the same issue on my server. You need to copy over the 1.16 files(Behavior_packs,etc) to your new server so your nether generates correctly.