Channeling:

The enchantment Channeling doesn't work when the enchanted item isn't in the mainhand when the projectile hits anything, e.g. when you throw a trident in Survival/Adventure Mode.

Recreate:

• Create new World
• give yourself a Trident and enchant it with Channeling
• set the weather to thunderstorm
• throw the trident at a mob in Creative and the enchantment works as it should
  throw the trident at a mob in Survival and the enchantment doesn't work at all

Custom Enchantment:

Custom enchantments with the "post_attack" effect also don't work when the enchanted item isn't in the mainhand, even if the slot is set to "any". You can swap a (cross-)bow with a custom enchantment to the offhand or fire it from there and the enchantment wont activate when the projectile hits a mob. This also doesn't work if the enchanted item gets swapped into a non mainhand & offhand slot or thrown onto the ground before the projectile connects.
You can also use a trident as mentioned above and the enchantment won't work.

Recreate:

• Create new World with the attached Data pack
• give yourself a bow and enchant it like this: "enchant @s custom_enchantments_test:hit_detection"
• hit a mob with the bow or shoot it and the enchantment works
  don't have the enchanted item in the mainhand when the shoot projectile hits a mob and it doesn't

Update:
included the right data pack version this time