Custom dimensions have no way for the player to sleep or set their spawn point. Both beds and respawn anchors explode when you try, no matter the world settings.
Phantoms can spawn in custom dimensions as long as they have skylight, yet beds still explode, meaning there would be no way for a player to stop them from spawning.

The only way to make either beds or respawn anchors work is to set the dimension type to the hardcoded presets minecraft:overworld (for beds) or minecraft:the_nether (for anchors). However, only one dimension is allowed to have these (see MC-184653), so custom dimensions cannot use them. Even if you otherwise replicate the overworld or nether it's impossible to get the expected behavior.

Steps to replicate:
1. Download the attached data pack
2. Add it while creating a new world with creative mode
3. /execute in custom:overworld_test run teleport @s ~ ~ ~
4. Try to use beds/respawn anchors and wait a couple of days to see phantoms spawn