I've been testing updating my server's world from version 1.17 to 1.18 pre-release 1 and have noticed that the dimensions do not load properly. This save is really two saves combined into one, meaning two overworlds, two nethers, and one end. The original world was created in version 1.14 and was updated throughout the years. Once updated, the default nether, along with second overworld, second nether, and default end were all inaccessible.

This was partially solved by defining a custom dimension in the minecraft namespace that overwrites the default dimension's configuration.  To make the default nether load once again, a custom dimension was defined as:

{
        "type": "minecraft:the_nether",
        "generator": {
                "biome_source": {
                        "type": "minecraft:multi_noise",
                        "preset": "minecraft:the_nether"
                },
                "seed": 3268689782729909057,
                "settings": "minecraft:the_nether",
                "type": "minecraft:noise"
        }
}

In order to load the second overworld, the biome source "type" was changed from "minecraft:vanilla_layered" to "minecraft:multi_noise". The second save's dimensions were defined in a separate namespace.

I would think this is an issue with the world updating feature not checking custom dimensions when updating and as a result, the outdated info causes the world to not load said dimensions.

UPDATE:

Starting in pre-release 4, this solution no longer works, the custom dimensions are working however the nether and end aren't.

UPDATE pre-release 6:
As of the latest pre-release this issue still shows up the same way as before.