The advancement description of "Uneasy Alliance" states that ghasts have to be brought from the nether, but can be obtained from killing a ghast spawned in the overworld

Don't worry about their descriptions, there are many more advancements with "wrong" descriptions. For example, it says form and mine a block of obsidian, when you can just find it in a chest. Point is, don't worry about how they are described, it's just the simplest walkthrough to completing it.

That description is correct for survival, where achievements are normally earned, making it correct-ish.

Reopening because issues such as MC-173756 and MC-182944 have been considered valid and even fixed.

Can confirm.

Can confirm in 21w15a.

Can confirm in 1.18.2 and 22w11a.

Can confirm in 1.19.2.

Here's a code analysis of this issue.

Code Analysis:

data > minecraft > advancements > nether > uneasy_alliance.json

{
  "parent": "minecraft:nether/return_to_sender",
  "criteria": {
    "killed_ghast": {
      "conditions": {
        "entity": [
          {
            "condition": "minecraft:entity_properties",
            "entity": "this",
            "predicate": {
              "type": "minecraft:ghast",
              "location": {
                "dimension": "minecraft:overworld"
              }
            }
          }
        ]
      },
      "trigger": "minecraft:player_killed_entity"
    }
  },
  ...

If we look at the above json file, we can see that the requirement to obtain the "Uneasy Alliance" advancement is to simply kill a ghast that's in the overworld. No checks are carried out to see if the ghast traversed to the overworld from the nether, therefore allowing this advancement to be obtained under circumstances that don't comply with the advancement's description.

You can even obtain this advancement by killing a ghast in the nether when the player is in overworld.

Stuck a ghast in a minecart and lit a tnt in front of it when you go back to the overworld could obtain this advancement.

This can be done in survival.

That is MC-251617.