Was you using a server API before like Bukkit/Spigot/Paper and then using vanilla server SMP? API breaks worlds into separate folders, using Vanilla will regen DIM-1 and DIM1 folders. Do you have world_nether and world_the_end folders? If so, using a server API mentioned above will reconnect them as they divide worlds to separate folders.
Cal J
Was you using a server API before like Bukkit/Spigot/Paper and then using vanilla server SMP? API breaks worlds into separate folders, using Vanilla will regen DIM-1 and DIM1 folders. Do you have world_nether and world_the_end folders? If so, using a server API mentioned above will reconnect them as they divide worlds to separate folders.