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.
Because you went from 1.14 to 1.15 then back to 1.14 on the same world that's your fault for not having extra backups.
1.15 changes things drastically and 1.14 cannot see 1.15 changes so you screwed your own world up.
You should have loaded a backup of your world.