Saiirod

Yesterday I managed to get a largebiomes map to work and keep generating largebiomes on a vanilla server with 1.16-pre5.

It seems OP tried to do the very same but failed in the original post and in the version at the time (Unsure if anything changed but it does work now), the way I got it to work was to generate the map with the seed I wanted in largebiomes in singleplayer (1.16-pre5, like the server), then use that save as the map for the server, with its server.properties aligned with the choice made to generate the map in singleplayer.

it seems to work atm and there is no issue with the world generator, it seems to only happen when creating a map from scratch.

Bug seems to persist in 1.16-pre2, on a vanilla server with largebiomes. However, like before, it work just fine in singleplayer

I noticed the same bug yesterday afternoon on version 20w22a, I couldn't generate amplified or largebiomes on a vanilla server. (Just like OP, tested in singleplayers and it worked just fine.)