Ever since I started this server the day of 1.18 release, whenever we go through nether portals we fall through the world (There are no blocks, only entities and particles) for a few seconds and then get launched back up and everything is normal. This happens when entering the nether and coming back into the overworld. 

It happens for me (the host) and all other players. The server has 16GB allocated (and my minecraft install has 8GB) because I have enough unused RAM and just want zero lag. Otherwise everything works fine and there is no other lag on the server. It happens with great internet as well (even though this shouldn't matter as I am the host).